Overdrive/A.X.I.S.

Surge Core
The Surge Core is exclusive to the Harvester. When the ship reaches 75%, 50% or 25% health, it charges up a huge shockwave that deals moderate Alien Damage. Upon ship destruction, all previous shockwaves are cancelled and a final shockwave is instantly discharged. The level I version is for NPCs only, while the level II version applies to player-owned Harvesters.

A very similar version is exclusive to the Altairian Mothership, which does Plasma Damage instead.

Note that these Overdrives are not affected by any bonuses.

Trivia

 * The Surge Core was originally considered as an item, the Pulsar Charger. It was first seen in Harvester fleets during the original Alien Decimation.
 * Because of this, Surge Core is the first and oldest Overdrive in the game, due to the ship predating the concept.