Brok Dalingar needed help to defeat a warrior's ghost near the old mine in the east of Liannon. The ghost had already slaughtered some adventurers who had tried to defeat it, so no one dared to go near it anymore.
The ghost would attack in melee combat and if being wounded, it would use lifetap spells to regenerate health. With some pressure the ghost could be overwhelmed easily though.
The ghost would dissipate but leave behind a ring and a sword. The sword was given to Brok as a proof of the ghost being defeated.
Brok would give an advise when it is safe to fight the ghost. Usually he'd say that from level 5 onwards, it's safe. But if you've summoned your rune hero companion and completed Path to Eloni, then you should be strong enough to deal with him.