How would this sound for Tirek: Unlike normal Tirek does not gain a Doom Token each time a Gate opens. Instead anytime an Investigator loses a combat against him he gains 2 Doom tokens, and the Investigator loses the use of one Ability for the rest of the game. He also gains 1 Doom Token each time the Terror Track increases."
There will be a wandering monster token of Tirek that the PC's can encounter. Flavor wise it's okay, but mechanically i feel it may be weak as the PC's will go all out to avoid him.