Demonic empire

So, for the event, three demons want to search out a cave for lost demonic relics / lore / dev etc (mim wants death dev iirc). I've got little to go on for the event itself other than "cave". So, I was thining of the cave leading to an old demonic lord's compound / manor. Something from before the cataclysm.

They'd be met by the lord's undead servant. A near perfect undead created during the height of the demonic empire. He'd initiate with them assuming they were delegates from the long lost empire as the Lord has not had contact since then. Once realising they cannot understand one another he activates an old runic thing to allow for perfect verbal translations (you hear and understand any language spoken to you). (just some fluff)

They enter the lord's manor one way or another, either through killing the servant or having him treat them as honoured guests. He mentions that his master had not been himself recently but he's sure that some fresh faces / his own kind would bring him out of his stupor. The servant doesn't seem all there in the head with his manner of speaking and his actions. If the servant is dead then they shall be free to explore the manor.

Ancient Demonic architecture litters the place and it clearly sized for their species and not humans, the servant would have been dwarfed by everything. Paintings along vast walls depict great cities and forges. Streets lined with the glow of mana as it practically falls through the air like cherry blossom leaves.

Hints of old lords from before the cataclysm are name dropped by the servant / are shown in various paintings. A painting of the demonic capital city from before the cataclysm is shown, looks great etc.
Now, how does the demonic caste / hierarchy system from their empire work? I was thinking of a triumvirate of demonic lords. One smith, one runescribe and one alchemist. They would keep each other in check in a rock paper scissors kind of deal and they would form a dominate position at the top that no other demon could ever hope of inserting themselves into. As should be fitting for leaders of a chaotic age.

The servant goes to fetch his master / disappears into an ornate door that seems far too large for him. Party can wait but the servant never returns. They are given free roam of the building. There's a library, the only place untouched by the servant. It is littered with dust, mould and disuse. Books in ancient agarthan can be found here but they would need translating before they could be used.
There's a workshop filled with other failed undead creations, still "alive" in their cages. Various abominations, things designed for specific jobs made out of the stitched corpses of humans etc. (Painting the picture that humanity was just another resource to use for the demons in their growth.)

They return to the main lobby and within it is something they may not have found the first time. A map of the old empire, in good condition but it is in an unreadable language and is shaped on land masses that were destroyed in the cataclysm.

When they go to check on the servant / enter the main chamber it leads into a ruined office. The desk has massive gouges taken out of it, all of the furniture is shattered beyond repair and littering the ground are the rotten corpses of less power demons and undead servants alike.

Crouching behind the desk and eating the remains is the demonic lord in question. He is barely coherent, his mind having regressed while his strength increased, turning into barely any more than a savage animal. The sight of the demonic party gives him a moment of pause before he snaps back into savagery.
They fight him and if / when they win his final words celebrate the rebith of the demonic empire (Which needs to be named).

I can shove in lore everywhere it's needed but this is a basic outline of the event and some of the lore plot points.