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During Kerrigan's resurgence, Abathur discovered the impaler strain after assimilating impaler colonies on the surface of Marek V, where the local hive cluster was under attack by the Dominion Prometheus Company. The impaler colony was a creation of the Overmind and an ancestor of the now-defunct sunken colony. After collecting the essence of the impaler colony, the impaler was formed from the hydralisk, and subsequently crushed the Dominion forces in short order. The impaler is able to launch tentacles that shreds through flesh, bone and armor.
It functions similar to a lurker with its ability to attack only while burrowed. However its attack is quite different, the impaler is only able to launch a single tentacle to an enemy that does massive damage. It has 11 range, and deals 20 damage to a single target, 45 to armored.
Impalers are better offensively compared to lurkers, due to having longer range and stronger damage versus armored targets (which includes any buildings). However, their lack of splash attacks means they can't fend off multiple foes nearly as easily.
Impalers also require a clear ground route to their targets to attack them. If there is a chasm separating the impaler from its target, the impaler cannot attack it.
Impalers are strong against marauders, firebats, thors, roaches, ultralisks, stalkers, and structures. They are weak against marines, zerglings, zealots, immortals, archons, any air units, and detectors in general. They are on par with siege tanks and colossi, but are out-ranged by siege tanks if detected. They also deal their bonus damage against hybrids, but will also be detected by hybrids even while burrowed.
Impalers originally had an ability named "spine lash" that dealt damage to units and stunned them for three seconds. Associated upgrades increased the range of spine lash by 50%, the damage it inflicted by 50%, and the stun duration by one second, to a total of four seconds.