If the caster makes a successful melee touch attack, the target of Harm has their Hit Points reduced. The amount reduced is based on caster level:

15 and under: 10 Hit Points per level
Levels 16 - 20: 12 Hit Points per Level
Levels 20 +: 15 Hit Points per Level

If the amount of damage is greater than the target's current hit points, that creature is reduced to 1d4 hit points. If the caster's level is less than the target's CR (Challenge Rating), only half damage is done.

This spell has a reverse effect when used on undead, healing instead of harming them (undead's Hit Points are increased according to the same progression by this spell). A successful touch attack does not need to be made when this spell is used against undead.