Minecraft wiki entry for Development_versions, associated text: Only spawns in the Nether.
Gives players "Wither" effect when hitting them.
Carry and rarely drop stone swords when killed by players.
Drops bones and coal.
Has a high chance to spawn from a skeleton egg if the player is in the Nether
Wither skeletons can replace skeletons on spider jockeys.
Spawned by using a spider spawn egg in the Nether.