Thorim's Sigil and its Heroic mode equivalent are a portion of the third step in the Algalon the Observer attunement quest chain. Completing this quest requires the defeat of Thorim in his Hard mode: Engage Thorim within three minutes of starting the gauntlet, adding Sif to the fight.
For more information on obtaining Thorim's Sigil consult the Archivum Console.
Thorim's not himself, <name>. He's come under a dark influence.
An evil presence under the guise of an illusion pretends to be his lost wife, Sif.
Ye can't let him be overcome by this impostor's spell. If he does, the location of his sigil might forever be lost!
Yer going to have to beat some sense into him and yer going to have to do it quick.
As always, the Archivum Console will give ye all the gory details... even the ones ye don't like.