From MTG Salvation Wiki
- Wither's effect applies to any damage, not just combat damage.
- The -1/-1 counters remain on the creature indefinitely. They won't be removed if the creature regenerates or the turn ends.
- Since damage from a source with wither is real damage, it follows all the other rules for damage. It can be prevented or redirected. When it's dealt, it will cause lifelink and other similar abilities to trigger.
- If the source of the damage is still in play at the time that damage is dealt, its characteristics are checked to see if it has wither. If the source has left play by then, its last known information is used.
- Damage from sources with wither affects players and planeswalkers normally.
The latest reminder text for Wither reads "This deals damage to creatures in the form of -1/-1 counters".
Cards that grant Wither
- Corrosive Mentor (black creatures you control)
- Fists of the Demigod (as long as enchanted creature is black)
Plane card that grants Wither
- Raven's Run (all creatures)