Well like fools the party attacked Yeenoghu and his 15 or so gnoll minions. They had surprise and elevation so they lead off with 2 fireballs from cover, which did nothing much to the big guy but wiped the gnolls out. Then some of the melee classes jumped down to ground level and using Legendary action charges Yeen engaged them. The fight took 5 rounds and when it was over 5 PC dead, the rogue having never moved to the same level fled down a tunnel that big guy couldn't fit down. The thing was they had him down to 60 HP and without a big round by Yeenoghu I think they may have taken him down next round. The 1/2 Orc Paladin was doing insane damage with smite crits. Knocked off a third of his HP in one round. Crits are far too crazy in this edition. I think if they had been one level higher they would have taken him down, or forced him to flee. Then again if I hadn't been playing him as fighting to the death since he is a demon prince I think he would have been forced to make a morale check and possibly flee as it was. Party was making saves just fine, they just ran out of HP since he hit all attacks but one. They had amulets making him attack at disadvantage but they were meaningless with his +16 bonus.