There's techincally no summoned witch, since it's only a heart, either Capt. stabs it and summons the Night Lord, which then when defeated drops Phantom's Longsword or you give the heart to the statue, granting you an Amulet of the Dark Whispers, which is really good for clerics and mages.
All-though, there is no really impact on the story in the current EK versions, maybe there will be later, but for now, when giving heart to the statue, Capt. dissapears, and you can learn that from the tavern rumors.
I just entered the icemist fort and got the quest and now who should i return the heart . I saw wiki and learned returning heart to statue summons a witch but returning it to captain kills the witch and i get sword .Btw I am a mage .