Hypnos

Hypnos, Greek God of Sleep.
Member of The House of Erebus.
First appeared in Season 1.
Was sent to kill Fionn Corvin after Fionn defeated Morpheus and refused to take his life.
Fionn and Hypnos had a tough hand to hand fight but when Fionn won, Hypnos didn't get back up. Fionn had killed him by accident.
Season 2
He was briefly featured as The Ghost of Christmas Past when Fionn had a Christmas Carol experience in 1985.
Fionn got to apologize for killing him, Hypnos said he understood, told Fionn they were both fighters, not very different at all.