Damaged Necrotic Shard

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

When all the Shadows of Doom are killed, the Damaged Necrotic Shard will despawn.