What is the name for a word or name that is used to replace another that is forbidden?

euphemism - the substitution of an agreeable or inoffensive expression for one that may offend or suggest something unpleasant; also : the expression so substituted.

It's more commonly used in contexts where the "offensive" term is so-called because some (or many) other people consider it rude/smutty or sacrilegious. But effectively the word being avoided is taboo, whether that's because of general perception or the edict of an all-powerful ruler.

An "alias" is a name that is assumed or used in place of a person's real name. It's usually adopted by the person themself, but by definition, is not the only use or assignation of an alias.

In some cases, use of a alias is meant to disguise the true identity of the person (as in your example), or in this example, is self-assigned:

When the American pop singer Prince had a recording contract dispute that would not allow him to make music under his own name, he adopted a love symbol as his alias, intentionally choosing an unpronounceable symbol since he intended to use his name again once the terms of the contract had expired.

There is a word 'hemiteleia'. Before you get too excited it is not even in the Oxford English Dictionary. But it is recorded by the following two authors:

Roberts, Chris (2006). Heavy Words Lightly Thrown: The Reason Behind Rhyme. Thorndike Press. ISBN 0-7862-8517-6. Bryson, Bill (1990). Mother Tongue. Penguin. ISBN 0-14-014305-X.

I know that Cockney Rhyming Slang is not the precise subject under discussion, but in many instances it is designed to replace a word that the speaker wants to avoid. The way it works is that a word like 'stairs' will be replaced with 'apples and pears'. Since 'pears' rhymes with 'stairs' the speaker will drop the pears. They will then refer to 'stairs' as 'apples'.

In the case of the word 'c**t', the rhyming slang is 'Berkeley Hunt'. So anyone wishing to refer to a woman's genitals will just mention 'her berkeley'.

And so as not to appear sexist about this I should perhaps mention 'cobbler's awls', and we (chaps) all know what a kick in the 'cobblers' feels like.

Now apparently this system of subterfuge is referred to as 'hemiteleia'. It, and Rhyming Slang generally, are extensively discussed in a Wiki article: see.

http://en.wikipedia.org/wiki/Rhyming_slang#cite_note-roberts-1

There is the concept of nom-de-guerre (war name) dating back to at least the early 18th century. In later usage

Noms de guerre were adopted by members of the French resistance during World War II for security reasons. Such pseudonyms are often adopted by military special forces soldiers, such as members of the SAS and other similar units, resistance fighters, terrorists, and guerrillas. This practice hides their identities and may protect their families from reprisals

These are a subset of pseudonyms which also include nom-de-plume (pen names) and stage names

Cryptonym:

OED:

A pseudonym or code name; esp. one given to a spy or to a clandestine operation.

1862 St. James's Mag. 4 490 For a short time he assumed several unobtrusive civilian cryptonyms.

1876 J. R. Lowell Dante in Among my Bks. 2nd Ser. 16 Only a cryptonym by which heretics knew each other.

The second example is probably closest to the one you mean.