Swan hunting, demon horde fighting, and marriage counseling: we’re back in the stories of Cú Chulainn, the hound of Ulster, after he won the war. He’s at a party, celebrating, but then his wife hints that she’d like one small thing, and that snowballs into him going to battle in fairy land against Senach the Spectral, shadow king. Really, though, if you’re a legendary hero, pretty much any todo list can end up with you fighting a shadow king.
The creature is Master Stoorworm, and he's still around. You can find him in the Orkney Islands and Iceland. Oh, sorry, did I say "in?" I meant "as."