Mob Essence - Alec-WAM/CrystalMod GitHub Wiki

Used to set the Advanced Mob Spawner's mob. To obtain Mob Essence just kill a compatible mob with an Empty Mob Essence in your off-hand and it will increase the kill count on the Mob Essence. Once the Essence is full you can activate an Advanced Mob Spawner to set the mob type. The kill count will decrease, so if you have to move the spawner you will have to fill up the Essence again to set the spawner's type again.