What is a long, complicated word for adding many unnecessary details to make a story seem more believable?

Perhaps the word you're looking for is elaborate.

: to expand something in detail; would you care to elaborate on that statement

Corroborate is another possibility that may better fit the part about believability.

to support with evidence or authority : make more certain

expatiate, from Dictionary.com

to enlarge in discourse or writing; be copious in description or discussion

Example: He expatiated endlessly on the theme of the joys of parenthood.

The word the OP is trying to remember may be expatiation, which could logically make sense for a climbing route that was the opposite of direct.

Embroider, from Dictionary.com

to adorn or embellish rhetorically, especially with ornate language or fictitious details: He embroidered the account of the shipwreck to hold his listeners' interest. (Emphasis added.)

What popped into my head was Shaggy-Dog Story:

In its original sense, a shaggy dog story or yarn is an extremely long-winded anecdote characterized by extensive narration of typically irrelevant incidents and terminated by an anticlimax or a pointless punchline. (Wikipedia)

It's not really a long, complicated word but at least it's not used so much any more.

• circumlocution: the use of many words where fewer would suffice, especially in an attempt to deceive.
• sesquipedalian: the use of long words for the sake of sounding important or detailed or meaningful.
• loquaciousness: overly talkative and using many words (probably not this one but I threw it in just in case)
• pleonasm: the use of more words than are necessary to convey meaning (e.g., see with one's eyes ), either as a fault of style or for emphasis.
• tautology: the saying of the same thing twice in different words, generally considered to be a fault of style (e.g., they arrived one after the other in succession ).
• rhetoric: language designed to have a persuasive or impressive effect on its audience, but often regarded as lacking in sincerity or meaningful content.

Those are ones I could think of quickly. Is it any of these?

There's a very relevant quote from the Mikado by W.S. Gilbert (of Gilbert and Sullivan).

Merely corroborative detail, intended to give artistic verisimilitude to an otherwise bald and unconvincing narrative.

I don't know whether you're going to find one word that encompasses your definition.

I like superfluous.

: Definition of superfluous. 1a : exceeding what is sufficient or necessary : extra; b : not needed : unnecessary. 2 obsolete : marked by wastefulness : extravagant. [Webster's]

"The superfluous details made the report practically unreadable, but it impressed the boss because of the amount of its size."

What about storytelling ?

Storytelling is the social and cultural activity of sharing stories,
often with improvisation, theatrics, or embellishment... http://encyclopedia.thefreedictionary.com/storytelling

OK, Perhaps Fabulate : To engage in the composition of fables or stories,
especially those featuring a strong element of fantasy.
http://www.thefreedictionary.com/fabulate

Periphrasis is a stylistic device that can be defined as the use of excessive and longer words to convey a meaning which could have been conveyed with a shorter expression or in a few words.

Note: It is very similar to but not quite the same as circumlocution.

SOURCE: https://literarydevices.net/periphrasis/

I suspect that the term you are seeking is a rhetorical device, such as hypotyposis. From The Century Dictionary and Cyclopedia, via Wordnik:

In rhetoric, vivid description of a scene or an event, as though it were present before the eyes of the audience; an oratorical word-picture.

Rhetoric is rich in terminology, and there are actually a number of plausible possibilities that seem worth investigation. Ann Vasaly mentions several in her Representations: Images of the World in Ciceronian Oratory:

rhetorical handbooks . . . advised [speakers] to use concrete details in order to create a "visual image" in the minds of his listeners. As mentioned in chapter 1, the technique can be found under a variety of names, and it is often identified with ekphrasis, enargeia, hypotyposis, diatyposis, evidentia, repraesentatio, illustratio, demonstratio, descriptio, and sub oculos subiectio. Further, it is subsumed under a number of theoretical categories, including techniques of the narration and of the peroration, aspects of ornate style, and figures of thought.

[Bolding added.]

Looking at some of these separately, either hypotyposis or enargeia seem likely. From Silva Rhetoricae (bolding added; note all definitions are at the same link; use search or the right navigation menu to go to the correct frame):

hypotyposis • ὑποτύπωσις

^{hy-po-ty-po'-sis from Gk. hypotypoein, "to sketch" (typos = "impression, form")
Also sp. hypotiposis
demonstratio, evidentia, adumbratio, representatio[,] the counterfait representation, word-picture}
Synonym for enargia. Lively description of an action, event, person, condition, passion, etc. used for creating the illusion of reality.

And for enargia:

^{from Gk. enarges, "visible, palpable, manifest" diatyposis, hypotyposis[,] demonstratio, descriptio}

Generic name for a group of figures aiming at vivid, lively description.

Ekphrasis might be a strong contender in its original rhetorical meaning; however, it is likely that any definition found in a standard dictionary today would contain the more specific, modern meaning, such as Oxford Dictionaries':

The use of detailed description of a work of visual art as a literary device.

(See Ruth Webb's Ekphrasis, Imagination and Persuasion in Ancient Rhetorical Theory and Practice for a discussion of how a more general device came to be associated specifically with art criticism.)

The Latin phrases look less likely to me, partly because I think they are similar enough to English words that you would probably remember them that way, and partly because they seem to be less commonly used in this specialized way. And sub oculos subiectio is not a single word.

Of all of these terms, hypotyposis seems most likely, as it is common enough to be included in various mainstream dictionaries with approximately the correct definition. One divergence from your description for any of these terms, however, is that they do not focus on unnecessary details, but rather consider vivid language to be useful and desirable.

While it doesn't sound as rarified as you hoped for, Amplification may fit the bill.

From the OED

Rhetoric. The extension of simple statement by all such devices as tend to increase its rhetorical effect, or to add importance to the things stated; making the most of a thought or circumstance.

and

The particulars by which a statement is amplified or an account exaggerated; the amplified or exaggerated statement itself.

As the first definition makes clear this is a definition related to Rhetoric, defined in the OED as:

The art of using language effectively so as to persuade or influence others, esp. the exploitation of figures of speech and other compositional techniques to this end; the study of principles and rules to be followed by a speaker or writer striving for eloquence, esp. as formulated by ancient Greek and Roman writers.

Wikipedia gives this definition of amplification:

Amplification refers to a literary practice wherein the writer embellishes the sentence by adding more information to it in order to increase its worth and understanding.

Which sounds very close to the OP definition.

If amplification is too pedestrian to be our target word, there may be related terms from rhetoric which suit better. Again from Wikipedia:

Amplification comes from the Greek word auxesis

OED gives Auxesis as:

Amplification: (a) a gradual increase in intensity of meaning (obs.); (b) hyperbole.

Wiktionary gives Auxesis as:

auxesis (plural auxeses)

(rhetoric) A rhetorical device whereby the subject matter is made greater, particularly 1. (rhetoric) Overstatement, hyperbole.
1577, Henry Peacham, Garden of Eloquence: Avxesis, when we vse a greater word for a lesse, or thus, when the word is greater then the thing is in deede.
2. (rhetoric, obsolete) Arrangement of a series in ascending order.

Since nobody proposed this yet, I'll just put it here, since I believe it's the most suitable, although OP has used this word in the question itself.

embellish

Sub-definition in Oxford Dictionary:

• Make (a statement or story) more interesting by adding extra details that are often untrue

And the corresponding embellishment:

• A detail, especially one that is untrue, added to a statement or story to make it more interesting.

Personally, I always see this word being used in negative context, which matches this second definition. Disclaimer: I'm a non-native speaker of English.

Amphigory: a nonsense verse; a rigmarole, with apparent meaning, which on further attention proves to be meaningless (Wiktionary).

This word is mostly used to describe a person embellishing and intricating some fact in a (generally) fancy way and complicated words with the end result being nothing established firmly.

I was thinking of the word exposition, but that doesn't seem to imply embellishment as strongly as you've indicated - it is a more general device. Then somewhere between reading the formal definition of exposition and your post:

The correct answer will be a long, complicated word not ever used in every-day speech. It could also be a non-verb that encompasses that basic idea

the idea occurred that perhaps the word you are looking for is expoundment.

[The closest I have here is a transitive verb definition for expound:]

2: to explain by setting forth in careful and often elaborate detail

[https://merriam-webster.com/dictionary/expound]

It certainly seems like a word that could show up as a play on escarpment in your context, but is not a word I have ever heard much of if at all.

Throwing out a guess since I haven't seen this mentioned:

Flosculation (n)

an embellishment or ornament in speech

The speaker's lecture was rendered laughable by ridiculous flosculations.

From page 71 of Erin McKean's Totally Weird and Wonderful Words:

flosculous: an adjective meaning 'like a flower' or 'flowery'. From a Latin word meaning 'little flower,' which also gives us floscule, 'something shaped like a little flower' or 'a flowery speech' and flosculation, 'speaking in a flowery way.'

Seems to me most answers are working on the "add details" angle. Let's try instead approaching it from the other side, "make real".

Well, immediately one word leaps out:

substantiate verb, to come into being. Too common, but otherwise "substantiating details" would be a fairly good fit for the definition given. This at least suggests that this might be a good approach.

evince, manifest, illustrate, concretize. All possibly related, but unlikely to be The One. Too common.

Another approach is to go along synonyms for 'credibility'.

facade, similitude, authenticity, fidelity, cogency, weight, credibility. Again, all are words I might use in a phrase to describe the OP's criteria, but all are a little too common, and don't quite, on their own, fit the bill.

Another thing I've been looking out for is terms that would look good as the name of a rock climbing wall: 1006a's 'enargia' is good, implying energy. My 'concretize' and 'facade' both fit the bill nicely.

And OK, this one's out from the far left field, but the first thing I thought of was:

Greeble or greebling - a fine detail added to a model to make it seem more realistic. I've never heard this used to describe a literary device, but in SFX and modelling it's in common use. And it's a pretty good description of how climbing walls are built. Frankly, if it's not the name of a climbing wall, it needs to be.

https://en.wikipedia.org/wiki/Greeble

It's a fairly niche and esoteric word. It means exactly what was described, just not in a literary sense. The only thing it lacks is the verbal complexity: it's a fairly simple word. And sadly, there are zero Google hits for "verbal greebling".

Finally, where others have suggested lists of literary tools, I'd recommend instead checking through a list of cognitive biases: https://en.wikipedia.org/wiki/List_of_cognitive_biases

The most obvious way to gain the definitively correct answer, of course, is to ask the people at the climbing wall what it used to be called.

I was trying to think of an overlap between a climbing wall and additional details in storytelling, and thought of the common magician's technique of misdirection, imagining certain magicians that tell long, detailed stories as part of a distraction to the audience.

From there, we can get a word that's long, not commonly used (outside D&D), and might be a name for a climbing route if someone is trying to be clever:

Prestidigitation

prestidigitation (plural prestidigitations)

1.A performance of or skill in performing magic or conjuring tricks with the hands; sleight of hand. My favorite prestidigitation was when he pulled the live dove out of that tiny scarf.

2.A show of skill or deceitful cleverness. His writing was peppered with verbal tricks and prestidigitation.

I'm fond of "fabricating", though it isn't long or complicated.

"invent or concoct (something), typically with deceitful intent."

To me, "fabricating" implies intentional embellishment of a story, though not necessarily to make it seem more realistic.

The first thing that I thought of when I read your description was "Purple Prose"

https://en.wikipedia.org/wiki/Purple_prose

Purple prose is text that is so extravagant, ornate, or flowery as to break the flow and draw excessive attention to itself

Perhaps the word you're looking for is aggrandise, also spelled as aggrandize.

From http://www.dictionary.com/browse/aggrandise

1. to widen in scope; increase in size or intensity; enlarge; extend.
2. to make great or greater in power, wealth, rank, or honor.
3. to make (something) appear greater.

A short, uncomplicated expression, and one that is a great name for a climbing route, is Sexed-Up.

Sexed-Up (Wikipedia) - Since 2003 it has been used in the sense of making something more attractive than it really is by selective presentation; a modern update to the phrase "hyped up"... The phrase gained currency in the United Kingdom in 2003, when BBC defence correspondent Andrew Gilligan reported that the September Dossier had been "sexed up", and that the intelligence agencies were concerned about some highly dubious information contained within it—specifically the claim that Saddam Hussein could deploy weapons of mass destruction.

I was thinking of obfuscate.
ob·fus·cate (ŏb′fə-skāt′, ŏb-fŭs′kāt′) tr.v. ob·fus·cat·ed, ob·fus·cat·ing, ob·fus·cates

1. To make so confused or opaque as to be difficult to perceive or understand: "A great effort was made ... to obscure or obfuscate the truth" (Robert Conquest).
2. To render indistinct or dim; darken: The fog obfuscated the shore.
   _{The Free Dictionary}

What you describe seems to fit the definition of circumvolution (see Dictionary.com):

a roundabout course or procedure, or an instance of such: The speaker's circumvolutions bored the audience to tears.

It is generally assumed that circumvolutions have the purpose to embellish a speech, or at least to provide details or justifications (though it's not technically part of the definition).

Extemporize?

I know it is a stretch. Even it it might be used in similar situations it does not fit your definition, but since there have been a lot of attempts I thought I'd try in the off chance that it was that one.

Long winded is a term often used to describe this. Some use the term 'diarrhea of the mouth'.

Prolix [adjective] (of speech or writing) - using or containing too many words; tediously lengthy.

source: https://www.google.com/search?q=define%3Aprolix&oq=define%3Aprolix&aqs=chrome..69i57j69i58.5068j0j7&sourceid=chrome&ie=UTF-8

Well "plausible" is used in everyday speech but it does fit the definition of a word making a story or narrative seem more convincing or believable. It would help if you provide a sentence with the needed word you need omitted.

My first thought was for "waxing lyrical" but I settled on discombobulated. However, I'm not sure whether this word can be applied to something which is non-tangible.

The cat was discombobulated until it learned its way around the new house (https://www.google.com/search?client=firefox-b-d&q=discombobulated+meaning).

My use of a fitting sentence might be that 'the story, though somewhat discombobulated, arrived at its anticipated conclusion'.