With the door now opened, step through into a small cave. Don't start gathering mushrooms yet- instead look through the doorway to your left and make sure you either kill or avoid notice by the two ghosts waiting there. The cave has, in addition to a lot of mushroom patches, a few trinkets and a coinpurse at the bottom of the pool of water- which is very hard to see into. Try sneak-crouching to get your camera level below the water's surface.
Move on into the burial chambers, ignoring the stairs up and going straight to the far end. Hang a left, but don't rush in too quickly- there's a tripwire here, attached to a ceiling-mounted battering ram. Oddly, although the trap can't fire again thanks to the trigger being broken, the ram re-sets itself on the ceiling.
Make your way past and you can see some salables and a chest on the slab table- but be wary. The chest, as you should be able to tell by looking, is trapped. Open it from the end of the table and watch the trap set the front of the chest on fire for a while (don't worry, nothing gets destroyed) and then collect your rewards after the flames go out.
Check the two burial urns as well, then return to the nearby stairs up. Move slowly and carefully- at the end of this tunnel is a porticullis, which will cut you off the moment you enter the room.
Stepping into the room, whose center is occupied by a bright blue glow like on the cairn below, prompts Halldir to appear in the center of the room, atop the grate. Halldir is a very powerful draugr, who wields both magic and a two-handed sword in his attempts to kill you. If this weren't enough, he can use a strange ability that teleports him to a random location in the room, and frequently summons additional draugr, both casters and not, to aid him in his fight.
