Necrotic Shard

Killing undead mobs spawned around the shard will reduce its health. When the shard reaches around 1% health, it will turn into a Damaged Necrotic Shard (turning from green to blue) and 4 will spawn around the shard to begin channeling magenta energy streams into the shard. If you talk to the Cultist Engineers, you can trade to kill one and spawn an elite.