Self explaitory, but here's the error:
Error loading class: com/github/L_Ender/cataclysm/entity/AnimationMonster/BossMonsters/Ancient_Remnant_Entity (java.lang.ClassNotFoundException: com.github.L_Ender.cataclysm.entity.AnimationMonster.BossMonsters.Ancient_Remnant_Entity)
I get it when trying to load my kitchen sink modpack, which used to work a few weeks ago, but after trying to load it now, the game wont even enter the first mod loading phase.

Nevermind, it was an addon mod, I'm an idiot.