Overkill

Subtract the enemy's remaining Life from the damage dealt by the killing Hit. If an enemy has 300 Life remaining and the Hit deals 800 damage, the Overkill damage is 500. A Hit that deals exactly the remaining Life has zero Overkill. Damage over time can kill an enemy but cannot create Overkill damage.

Only the excess from the killing Hit counts

Overkill is calculated from one Hit that kills the enemy. Earlier Hits do not accumulate into the Overkill amount. With 300 Life remaining, a 250-damage Hit does not kill, while a later 800-damage Hit creates 500 Overkill damage.

Damage over time cannot Overkill

Overkill requires a Hit. Ignite, Bleeding, Poison, and other damage over time can deliver the killing damage, but that damage does not produce an Overkill amount for Herald of Ash or Trampletoe.

Overkill has no effect unless another effect refers to it

The game records excess Hit damage when the enemy dies, but the excess does not strike nearby enemies by default. A skill, item, or other effect must explicitly convert or use the Overkill amount.

Sufficient Overkill can require a separate threshold

Herald of Ash requires Overkill damage equal to at least 20% of the killed enemy's maximum Life. An enemy with 1,000 maximum Life therefore needs at least 200 Overkill damage. This threshold decides whether Herald of Ash triggers; it does not redefine whether a smaller positive amount is Overkill.

Herald of Ash uses only qualifying non-Herald Attacks

Herald of Ash checks kills from non-Herald Attacks. When the 20% maximum-Life threshold is met, it creates a 1.2 metre explosion that Ignites nearby enemies for 3 seconds. The base Ignite contribution scales from 16.67% to 23% of Overkill damage with gem level, and quality adds up to 5 percentage points.

Trampletoe converts part of Overkill into nearby damage

Trampletoe deals 30% of the Overkill amount to enemies within 2 metres of the killed enemy. PoE2DB identifies the underlying modifier as Physical damage. The boots use the size of the excess killing Hit, so stronger one-hit kills create a larger nearby hit.

Ice Crystals cannot apply these on-kill effects

Since Content Update 0.2.0, player-created Ice Crystals cannot apply on-kill effects such as Herald of Ash or Trampletoe's Overkill explosion. Destroying an Ice Crystal is therefore not a valid shortcut for these triggers.