MortalPlague
Adventurer
Glancing at your stat block, Tharizdun seems well-built (the triple actions will be especially potent), but I wonder if his damage output will be high enough? I'd be tempted to bump it up by half, or maybe even double it.
When I ran a group of five PCs against Ogremoch, Lolth, and Vecna (thread here, I didn't manage to down a single PC. Not once. And they all had contingency things that triggered "the first time they die during a day"...
The Lolth fight in particular had the goddess herself hitting for 4d6 + 24 damage and ongoing poison damage (20, I believe). With threatening reach 3. Then she had two gods fighting alongside. Vhaeraun had two attacks as his basic attack, hitting for 4d6 + 20 and 4d6 + 15 damage, and adding ongoing 10 poison damage on top of that. Then Selvetarm also had two attacks, hitting for 3d6 + 18 and 3d6 + 18, with the kicker "The target cannot gain hit points until the end of Selvetarm's next turn" and "The target loses resistances and immunities until the end of Selvetarm's next turn".
All that nastiness, and no PCs went down.
When I ran a group of five PCs against Ogremoch, Lolth, and Vecna (thread here, I didn't manage to down a single PC. Not once. And they all had contingency things that triggered "the first time they die during a day"...
The Lolth fight in particular had the goddess herself hitting for 4d6 + 24 damage and ongoing poison damage (20, I believe). With threatening reach 3. Then she had two gods fighting alongside. Vhaeraun had two attacks as his basic attack, hitting for 4d6 + 20 and 4d6 + 15 damage, and adding ongoing 10 poison damage on top of that. Then Selvetarm also had two attacks, hitting for 3d6 + 18 and 3d6 + 18, with the kicker "The target cannot gain hit points until the end of Selvetarm's next turn" and "The target loses resistances and immunities until the end of Selvetarm's next turn".
All that nastiness, and no PCs went down.