All those undead and Orcus have to do is get PCs below 100 hp and *pop* *pop* *pop*. They all die. Without saves. By the time the party can get to him, it might only be just one PC who can attack him. And one PC can't do 450hp of damage in a round. A monster doesn't have to have a lot of hp when it's near impossible to attack him. You gotta figure half the party might be incapacitated right off the bat between his undead minions' special powers (elder mummy, lich, etc) that require tough saves every round.
The undead minions will be the real problem. The "no saves if you're below 100 HP" aspect isn't an insurmountable problem. If the PCs have done their research (big "if" at some tables) and know what they're facing, you can negate PWK with any of the following:
1.) Death Ward in advance to negate PWK completely.
2.) Aid V or better in advance (for 25+ HP) + Mass Heal during combat, in order to not be below 100 HP.
3.) Counterspell during combat. Lore Bard with Enhance Ability (Charisma) will succeed 78.5% of the time according to my Monte Carlo sim.
4.) Readied action: Revivify to bring killed character back to life at 1 HP.
Honestly I think #1 would be more than sufficient. If Orcus is burning an action to cast PWK, and it does absolutely nothing, that's a win for the PCs. He can try again next round and maybe succeed (if the PC is still below 100 HP), if no one Counterspells him, but if he does so, yet another character can just Revivify that PC. Frankly that looks to me almost less effective than just relying on direct attacks like his mighty tail, which doesn't have to play the HP-guessing game.
(BTW, remember to pre-cast Protection From Poison on everybody to cut down on his tail attack's poison damage.)
IMO, Orcus's stats are weak, not so much from an in-combat perspective as from an RP perspective. As a combat threat I'm fine with him, but he's just kind of boring and gear-dependent, especially outside of combat. That doesn't mean I wouldn't use him, as a DM--it means that I wouldn't respect him, as a PC. He's just a gigantic one-trick-pony Demon Lord who can't even teleport. The
only thing he's got that truly scares me is probably an unintended loophole: it's the fact that the 500 HP of summoned undead have no duration, so apparently he can conjure infinite numbers of liches over time, and liches
do scare me since they're wizards like me, when they've had time to think and prepare.
Edit: I just checked the DMG, and it's not accidental. Infinite liches is deliberate. Huh.
What
really scares me is a wizard who's owned a Tome of Clear Thought for 10,000 of the last 25,000 years.