Abyssal demons are spawned from portals as part of the Slayer station of the Winter Chill Off. After spawning, they walk towards one of the two nearby signs. Players interacting with that sign will then instantly kill the demon, gaining Slayer experience. This does not grant experience in any combat skills, nor does it count as a kill for the purposes of Slayer assignments or the kill count in the beasts interface. If there is no player interacting with one of the signs, the demon will keep walking for a time, before vanishing.
The update history project is a work-in-progress – not all updates to this topic may be covered below. See here for how to help out!