Thief in the Myst (The Master Thief #2)

Jack Myst infiltrated the Thieves Guild to avenge his mother’s murder, but never expected to find a home. Now a master thief, he grapples with his new identity as he unravels the secrets of the fallen Guildmaster. The trail of questions will lead to a shocking truth, that his foe is a remnant of an ancient race, the very devil of Lumineia.

Skorn.

Seething with hatred and cast out by the thieves, Skorn is not without allies. Forcing an assassin into service, he journeys to the forgotten fortress of Margauth, where a cult worships him as a god. With such an army at his command he seeks a return to power, and two keys that unlock a legendary vault.

The Vault of the Eternals.