You'll have to wade through quite a few critters to get to the entrance because there is no direct path to Endusal. I prefer to levitate over the mountains and bypass all of that nonsense because it's a lot of fights for not much loot. Holding down your "sneak" button before entering would be a good thing because there will be a Dremora Lord directly in front of you when you enter.

Down the stairs to the right or left (they both lead to the same place) are a Daedroth, Dagoth Goral (an Ascended Sleeper) and a dead adventurer. The doors behind the Dremora Lord are neither locked nor trapped, but behind them you will find Dagoth Mendras (an Ash Ghoul) and an Ash Slave. The desks in that room are the usual Dwarven junk, though two of them have Dwarven Coherers (like you really need those at this point). The doors out of the room are neither locked nor trapped.

You'll find Dagoth Endus in his study (he appears to be a bit of an amateur alchemist when he's not busy terrorizing the citizenry).

Dagoth Endus (30th level Ash Vampire): 300 HP/500 MP, no weapons, no armor, 80% combat, 100% magic, 80% stealth

Aside from the typical junk you find in Dwemer ruins, you'll find a couple of potions, a lot of ingredients, some alchemical apparatus, Kagrenac's Journal (which you'll need if you're going to have Yagrum Bagarn activate Wraithguard), "Silence" (increases Illusion skill), an Ebony Broadsword, a Bonemold Cuirass and Glass Boots.