Acolyte's Guise is reward from the quest The Better Part of Valor, this is the last quest on The House of Rituals Chapter (Necrolords Campaign). If you complete side quests in the House of Rituals you will unlock a customization option for your Necrotic Acolylte disguise:
A Fatal Failure > Discarded Grimoire.
Casting Doubt > Sorcerer's Blade.
Regalia de Rigueur > Amethystine Dye.
Here you can buy Acolyte's Guise Toy Boost for EU or US Servers.
You'll get guide for completing Acolyte's Guise Toy Boost
