I think F.E.'s answer here is very good and complete--the best overview on the case of fused relative pronouns that I've seen on this site.
But the question is marked as a "duplicate" of two questions with answers that in my opinion don't explain the subject anywhere near as well, if at all.
This one is about fused relative pronouns, but it deals with a vocative expression containing a nominative relative pronoun, not a very common situation. The answer also doesn't mention the complications of sentences like "(?) [Whomever he marries] will have to be very tolerant" (which F.E. quotes from Huddleston and Pullum's CGEL).
This one deals with a similar situation, but the answer is much shorter and doesn't have references.
I actually found one more question, from 2011, that I think covers the same subject and has some interesting answers: Should I use 'whoever' or 'whomever': "I will kill ___ despises me." ?
So, what I would propose is marking "Put me in touch with whomever created it"? as a duplicate of Should I use 'whoever' or 'whomever': "I will kill ___ despises me." ? and merging the answers (since F.E.'s answer is mostly about the general topic, it can easily be edited to address the earlier specific question).
What do people think? Are there any more useful posts about the case of fused relative pronouns that I've missed?