1 : of or relating to the period before the flood described in the Bible
2 : a : made, evolved, or developed a long time ago
b : extremely primitive or outmoded
3 : In the tabletop role-playing game Vampire: The Masquerade the Antediluvians are the early vampires that lived in the pre-flood world and the supposed progenitors of the original clans.
Okay, the first few definitions are useful, but I’m reasonably sure this one’s in here for the third “definition”. Not all campaign settings have a flood mythology, but any that do can use the word in the same fashion as we do. The second definition is a little odd, and I’m not sure I’d use it in any write up or story. The Vampire reference though, that one is handy in any game that may use vampires as a plot or monster. The Antediluvians in V:TM are powerful, ancient and scary. They’re the mythic great wyrms of vampires. If you use or reference them, do so in that fashion. Ancient evils, that should be left be unless there’s no other choice.