WoWDB

Molthor is back with a vengeance. He is the only rare on the Isle of Thunder that has two spawn locations. The first spawn location is on the overlook above God-Hulk Ramuk, and the second spawn location is inside The Swollen Vault, after the tiles you have to jump for  Platform Hero. In either location, he cannot be accessed until the server has progressed to Stage 5 by taking control of Stormsea Landing, which means that he takes longer for your server to unlock than any other rare. To defeat Molthor, you must quickly move out of his cone-based AoE and quickly kill the 10 tiny bugs he summons.

Molthor's primary threat is his AoE-cone ability  Thunder Crush, which hits for 200k+. It only has a 1.5 second cast, and Molthor can make you  Dazed beforehand, which makes it nearly impossible to avoid the hit. Every 25 seconds or so, Molthor will cast  Horrific Visage, fearing you around the area. He will also continue chasing you, often causing said Daze problem above, and can even begin the cast on Thunder Crush before the fear effect ends, which also makes it nearly impossible to avoid the hit. Lastly, Molthor occasionally summons 10 Scarab with  Scarab Swarm. These adds have very little HP but can take off huge chunks of your health if you ignore them, so kill them fairly quickly.

Same thing but different way of saying it - you say "completed Stage 4", I say "progressed to Stage 5". I think the "progressed to Stage 5" way of saying it is easier to understand because the in-game map must say "Stage 5" for this rare to spawn, but it is arguable.