A sigilist is an NPC class based on the adept. The sigilist can be either arcane based or divine based, but must research or find their “spells” as a wizard would. (Most learn from the base “adept” list).
Sigilists do not cast spells as a normal adept. They may only use their spell knowledge in the form of runes created by the inscribe rune feat, (see FRCG pg 36) which they gain at first level. Sigilists do not have the summon familiar ability.