Thórth has fixed the broken key, and wants you to gather allies and find and defeat Arnmórth the Exacting within the Icereave Mines.
'Right! That ought to just about do it. The key be fixed.
'One of my cohorts caught an' interrogated a Dourhand just a little while ago, and found out that they're bein' led by a right vicious dwarf, name of Arnmórth the Exacting. He's the greediest Dourhand since Skorgrím: thinks o' nothin' but gold and how to keep it. Ye'll need help defeatin' him, just to warn ye: he's likely to have plenty o' guards.
'Now, take this key, find his quarters, open that door, and give 'im what for! ...And bring the key back when yer done -- once the Icereave Mines be ours again, we'll be wantin' to use that room.'
Find the inner office within the Icereave Mines. Bring allies.
Return to Thórth at Zigilgund with the key, and report your defeat of Arnmórth the Exacting.