http://pastebin.com/NC9jEkDB

Upon dying it causes this crash. Narrowed it down to Blood Magic v1.2.1a-1.
Also tested with 1.2.1-1; Same crash.
Tested by itself and with modpacks.
Able to duplicate multiple times.

EDIT: Apparently Blood Magic doesn't like it when you configure Potion IDS.... fixed on recreating the config.

Yeah, the issue is that you changed the potion ID when a mob already had that potion effect on it. Also make sure it is not below 0 or above 255. I'll wait to make sure everything is good before closing. :P

I changed the potion IDS to 220-235 in the configs. But the thing is, Each time I change configs, I delete the current world and create a new world. I haven't reused a old world for testing.
So I haven't the slightest idea why It would cause that.
But all in all, as long as the IDS aren't changed, the crash doesn't happen.

potion effects on an entity are written to the NBT as bytes. then Minecraft loads them as signed bytes, because it expects them to be 0 - 31. The only fixes that I can suggest is to make the potion array have a length of 128, or use ASM to make it load the potions from entities as unsigned bytes