The Sigil of Ahriman (G1: The Guardians)

An artefact of evil from antiquity is in the hands of a madman, and only the Guardians can oppose his insane schemes!

Against all precedent, a stranger is allowed into the inner sanctum of the Guardians. This man brings news of something that threatens the human race with eternal slavery. An artefact has been stolen, the Sigil of Ahriman. Now, it is in the possession of an unknown person, and there can be only two reasons for that - to tap its power, risking freeing Ahriman, or to deliberately bring the ancient god of Darkness into the world again.

G1, devastated by their last mission, must face this new challenge and win. Defeat is unthinkable.

A novella.  Being Part 3 of G1: The Guardians.