It’s rare, but.
At times a spirit of an unstable soul, has no clear element and a mutation occurs.
An acid spirit that behaves like a tumour, damaging the cells in minutes instead of years.
Moving like it does like it was a toxin.
Moving to the nearest unclean part.
Likely you lost al senses in that arm of yours, already.
Slash
Sorry about the limb.
Regrettably, this had to happen, the mutation has to be locked in the wall, where it belongs.