Huginn is somewhat hidden. Bring a see invis and go east and down from Muninn. Open the chest and there will be an invisible key inside. Go wu2wn and you will be one east of the door to Huginn's room. He will store soulshards for you.
Give soulshards to Huginn he will store them for you. You can get them back by asking him retrieve.
Once you have given Huginn 10 soul shards he will ask you for an item.
A drop of Y'mir's blood. - obtained by killing Ymir's Abomination
give the drop to Huginn and receive the mantle.