The trinket is a quest reward from Impsy, a quest giver in your Garrison at the Tavern. Impsy is not there every day but comes and goes randomly. Complete The Void-Gate quest to get the Manual of Void-Calling trinket.
This trinket will summon a void creature. Use: Summons a Bound Voidcaller to fight for you for 10 min. (1 Hour Cooldown) – Requires Level 35.
“The words are written in an ink so dark it seems to pull light away from the pages.”