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A symbiote is a zerg creature that attaches itself to larger organisms. They provide defensive capabilities for their host creature, and are able to generate an extra outer carapace and defensive projectile spines for the creature they are attached to. To date, only Abathur has shown the ability to generate symbiotes.
Symbiotes are available in Co-op Missions for Abathur once the player reaches level 10 with him. Symbiotes appear permanently on all brutalisks and leviathans spawned by Abathur, and cannot be controlled by the player. These symbiotes will automatically generate a 200 hit point carapace shield for their host creature once they are attacked, with a short cooldown. In addition, the symbiote will add an additional spine attack to their host creature, which will launch a projectile that does moderate 20 damage to any hostile unit in a range of 7.
Heroes of the StormEdit
Symbiotes appear in Heroes of the Storm as an ability of Abathur. Abathur will put a symbiote on a target hero, and use abilities to augment their offensive and defensive power. The spine and carapace abilities of the symbiote appear as abilities for Abathur.