Negative level

Each negative level gives a creature a -2 penalty on attack rolls, saves, skill checks, ability checks. Also inflicts a -1 penalty to effective level (affects things like spells and abilities which rely on caster level or character level). Negative levels stack.

If the target has as many negative levels as its Hit Die or levels, it dies.

Negative levels also affect maximum hit points. For monsters, each negative level reduces hit points by 10 %. 

Prevention

 * Warforged are naturally immune to negative levels from spells.
 * The Paladin HotD II PrE provides negative-level immunity.
 * Wizard Pale Masters in undead form are immune to negative levels.
 * Death Ward

Removal

 * Resting (at a shrine, or after a quest completion) cures all negative levels.
 * Negative levels wear off naturally, at a rate of 1 every 2 minutes.
 * Restoration restores a single negative level.
 * Greater Restoration purges all negative levels.
 * Cleric Radiant Servant Bursts restore 1d4 negative levels.
 * Shard Trinket of Greater Restoration.
 * Regenerate restores 4 negative levels over a short duration.