[Skarg]

All it takes is someone researching a variant spell and having the GM allow it.

(Even if that someone was an NPC, centuries before play started.)


Or maybe it happens the same day that play starts, the spell is very powerful, and the campaign is about what happens as that spell starts getting used...

(A big part of the "Thail" TFT campaign I played in featured some very strong not-in-Advanced-Wizard necromantic spells.)