In Out of the Abyss, Orcus has a number of Lair actions. One of those Lair actions has a confusing point:
Orcus's voice booms throughout the lair. His utterance causes one creature of his choice to be subjected to power word kill (save DC 23)
It is stated that there is a save with a DC of 23 but no specific type of saving throw. The spell it references doesn't have a saving throw either. Another of his Lair actions is properly specified as a strength saving throw, but not this one. What kind of saving throw is needed and how are we supposed to know?