There is an unofficial module on DMsGuild named "Shield of the Hidden Lord" by M.T. Black – who is one of the writers on the official adventure Baldur’s Gate: Descent into Avernus, so I assume its lore is correct.
Pages 7-8 of the "Shield of the Hidden Lord" PDF include the descriptions of 4 frescos on the walls of area T2. These paintings reportedly depict Gargauth (presumably different key aspects or events relating to Gargauth).
What events or aspects exactly are depicted in each of the following frescos?
First Fresco. A handsome sage in amber robes sits on a throne with a pile of scrolls next to him. He is speaking to a group of kings and queens, who listen to him with interest and respect.
- I suppose this is just an illustration of Gargauth’s charming abilities. Is that the only meaning?
Second Fresco. The sage in amber robes looks at a city in the background. The city is burning, and soldiers fight in its streets. In the foreground are two snakes, twisted together.
- What city is depicted here?
- Who are the soldiers, and what organisations do they belong to? Why do they fight?
- What is the meaning of these snakes?
Third Fresco. The sage in amber robes rides a blue dragon over a dark forest. In a clearing in the forest is a unicorn with a broken, twisted horn.
- I found this dragon’s name: Rathguul. Is that right?
- Why is there a unicorn?
- A twisted horn is Gargauth’s holy symbol. But the module says that Gargauth’s holy symbol is a Knight of the Shield badge. What is right here?
Fourth Fresco. The sage in amber robes wields a rapier and fights a shadowy figure in dark armor. The shadowy figure wears a jeweled gauntlet.
- Why a rapier?
- Who is the figure? Bhaal?