Mostly if your going with the Celtic or the Norse mythological Berserker / Warp Spasm effect you are looking at strength, toughness and speed powers with a human form (+1 for only when in trance mode) as Berserker's are physically stronger, tougher and faster when in berserk mode. Some mod of Demonic Co-Pilot also makes sense though if you want the character to be playable I would just do it with aspects.