The name or term "Dweller in the Depths" refers to more than one character or idea. For a list of other meanings, see Dweller in the Depths (disambiguation).
We finally learn the truth about Maccadam and, as we totally called, he's Alchemist Prime, taking inspiration from the idea put forth in Transformers: The Ultimate Guide that the old barkeep was one of the Thirteen Primes.
Real-world references
The Dweller, now a monstrous unknowable tentacle wielding monstrosity, has some heavy Lovercraftian vibes. Specifically he calls to mind Lovecraft's most famous creation Cthulu.