A regeneration potion corrupted by a wither rose can inflict a far more potent form of withering.
A wither rose by itself can be used, but a far more effective method is to distill the wither roses in strong alcohol to concentrate the effect. This causes a 3-fold increase in the speed of the withering, and can be devastating when used as a splash potion in battle.
The scent of the potion and the speed of the effects are very good at confusing the enemy. Use dangerously.