What does bone desease do?

your bone gets diseased :D

U lose hearts

You could get inflicted with Weakness (2). However, I'm not sure if it changed to the Wither effect, as I haven't met Bone Matters lately, though I remember the description now said something like “… or inflict the Wither effect on your opponent …”

It's the wither effect. I fought a bone matter yesterday, so I can confirm that