Movarth's Lair is a cave located in The Elder Scrolls V: Skyrim.
The entrance is unremarkable. Once inside the lair, the Dragonborn will be greeted by a couple of frostbite spiders. After disposing of the spiders, a passage to the south leads to an open room that offers three paths from which to choose, as well as a Vampire's Thrall to battle. The right and center options are dead ends but offer some loot. The left tunnel leads to a chamber with a hole in the ground and another Vampire's Thrall to fight.
Continuing through another tunnel will take the Dragonborn to Movarth's chamber, where a master vampire is accompanied by two guards. After he has been dispatched, the Dragonborn can continue east up a path that leads to a side chamber that's occupied by a thrall. Behind the thrall is a chest with leveled loot and a bookshelf against the south wall (near the Alchemy Lab) with the book The Aetherium Wars. There is another path to the south that leads to a room with a thrall, an Ancient Vampire, a chest up a wooden ramp to the right (west), and a dirt ramp up to the left (east) that leads through a twisty passage and eventually leads to an opening high above the dead end room to the right from earlier.
If the lair was cleansed before talking to the Jarl and beginning the quest Laid to Rest, the master vampire is not Movarth, but he'll be there after the quest is started.