Also, nothing strikes fear into a party quite like knowing the BBEG has Power Word Kill in their back pocket. No save, no spell attack, no change to interrupt. They just tell one of your party members to die, and the party member keels over dead. Hopefully you have a 9th level spellcaster who has counterspell prepped and is willing to upcast to prevent it.

It can force the party to come up with some pretty elaborate traps just to bait the PWK prior to the fight. Since you can only use it once per day, the goal is usually to force them into a fight with something sacrificial (like a golem or a troll,) bait them into use the PWK on that creature, then rush in before they can escape.