This happens to me if I exit the Slayer form before I start taking damage as well as afterward. I'll take 5, 10, 25, 50, and then 1500 Magical damage. The only thing that can stop this is ctrl+R on CHARNAME. It seems to get rid of the effect that causes the magical damage to accumulate. Even then, the Slayer Change ability does not return after resting or even after 24 hours.
CURRENT BEHAVIOR: Exiting Slayer Change still causes Magic damage to be inflicted in increasing amounts on CHARNAME. EXPECTED BEHAVIOR: The accumulating Magic damage should cease once End Slayer Change has been casted.