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.
I think you may be on to something. Knight of the Shield or Gargauth in disguise would make perfect sense for the symbol that Gortash wears since this is the description of Gargauth: "He was a clawed and fanged monster with jagged shards and horns that jutted from his face and back".