Originally Posted by KillerRabbit
My guess would be that he worships Gargauth, as the clip showed us, the alliance is a shaky one. He's Lawful Evil but Gargauth focuses on corruption and betrayal instead of tyranny. Of course Asmodeus is also a possibility.

He might be a knight of the shield

https://forgottenrealms.fandom.com/wiki/Knights_of_the_Shield

Which would explain his rise to power. In decent to avernus the party could choose to take the shield to hell with them and doing so would free Garauth from his prison. Looks like Larian chose that ending.

The symbol of the Absolute is a combination of the icons of The Dead Three. There are three lieutenants, each of whom thematically relate to one of the Dead Three. It’s Bane.

As to the OP- maybe he has a giant Bane tattoo across his entire back. 🤷🏻‍♂️