MonsterEnvy
Legend
Well we'll see how it goes. My player's characters seem to be able to dish out a lot on their turns (they work well as a team), so unless the monster has some staying power it can all go their way quite quickly. The Demon Lord stats in the OotA appendix seem quite low.
I prefer the abilities of the OoTA version. But you could always just give Yeenoghu max hp for his Hit die. That will increase his staying power by a lot if you are worried about that.
For example that is 480 hp.