I’ve not seen this wee buglet in The Elder Scrolls V: Skyrim mentioned anywhere else, so I thought I’d make a note of it here.
Halldir’s Cairn is the location of a couple of quests, so I’m led to believe. I was there to Find Rjorn’s Drum but you may find yourself directed there to collect something else.
I found a few bugs on this quest mentioned around (I searched as I was having trouble, as I’ll describe) – one where the pillars don’t work and the solution is to reload the area, and one where Halldir spawns invisibly in triplicate. This one however is on trying to kill the last of the Halldir copies. I’d knocked him down inside the light from the cavern below, on top of the trapdoor. I’m not sure whether the location of the collapse is relevant, or if it’s because my weapon is enchanted with paralysis – but a body was there. It wasn’t dead – it was showing about 40% health, but no amount of further bashing with my war hammer would kill it. It also didn’t call itself Halldir any more – it was listed as Draugr Scourge or similar.
I was baffled. I couldn’t kill the last Halldir (or loot him for his staff), and I couldn’t open the trapdoor to escape. The way in was blocked – Lydia was trapped behind iron bars there, along with her summoned Atronach, jeering redundantly at this semi corpse in limbo.
Having no idea what to do, I consulted the internet, but came up with nothing.
In the end, I just shouted at the body with Unrelenting Force. This seemed to get it back into it’s fighting cycle, and stand up so I could kill it. I guess it was the death that released the trapdoor.
So, if you find yourself with a troublingly undead enemy that can’t be shifted, try shouting at it.