Trying to determine if this sentence needs a comma; in my head it does, but I can't articulate why

I think there are two different problems with this sentence that are leading in opposite directions.

On first reading, the sentence appears to consist of two clauses:

• His neck took to [something which] had become apparent
• [the thing it took to was] its characteristically unnatural bend

As another answer says, these would most correctly be separated by a colon.

However, the intention seems to be that what had become apparent was not the bend itself, but the fact that it was characteristic. So we could try moving the "characteristic" into the first clause, to reunite it with "apparent":

• His neck took to [something which] had become apparently characteristic
• [the thing it took to was] an unnatural bend

But this is still unclear: is it "become ... characteristic" + "apparently" or "become" + "apparently characteristic"?

We would therefore be better moving "become" and "apparent" into a third, parenthetical clause:

• His neck took to something which ... was characteristic
• [the fact that it was characteristic] had become apparent
• [the thing it took to was] an unnatural bend

Re-constructing the sentence, we might say:

His neck took to something which, it had become apparent, was characteristic: an unnatural bend.

The sentence is definitely wrong. It should read

His neck took to what had become apparent was its characteristically unnatural bend.

And a comma anywhere in there would be wrong.

If you really want commas in there, you can do it like this:

His neck took to what, as had become apparent, was its characteristically unnatural bend.

But that sounds even clunkier to me.