[@Cu Chulainn] Capall Bhais - Harbingar of Death seems a bit broken. Unless they're being used on someone with high magical protection they're pretty screwed. Anyone who uses a notable weapon has their true name revealed quickly, and if their not a saint like Jeanne, or a Saber they need to pass a high luck check. It may not be fun to have a servant who can't talk outside of his mental connection either so I'd like for your too keep that in mind.

I'm not sure why you don't just go with the sleepy hollow headless horseman as they'd essentially just conjure up a reality marble of sleepy hollow which in itself doesn't disadvantage the enemy but buffs him up as well. He'd keep all the general ability besides divinity as well.