First off, this might very well be caused by Ars Magica 2, but, in the off chance it isn't, I figured I'd post this here.

Issue on the AM2 GitHub: Mithion/ArsMagica2#352

--more or less a repost of the above link below--

Mod List: (The Mad Pack 1.1.0)

-Ars Magica 2 v1.1.1c
-Minecraft Comes Alive 3.5.4

For a full list, see: http://www.atlauncher.com/pack/TheMadPack/

Steps to Recreate:

1. Spawn in Minecraft Comes Alive villagers until you get a guard (or any one with a sword)
2. Spawn in an AM2 Dark Mage
3. The game should crash as soon as they hit each other.

Other Notes:
-Tested with a few other AM2 mobs (Elementals, Hecate, Water Guardian), this only happens with the Dark Mage, potentially with the Light Mage if anything ever causes them to aggro each other.

Pastebin of crash:
http://pastebin.com/r9GJP1xb

Definitely caused by AM2.