Ur-ghast Spawner is a technical block used by the Twilight Forest mod for spawning in the Ur-Ghast. When the player gets near, it will remove itself and a Ur-Ghast will spawn in its place. This is very similar to how the other boss mobs of the Twilight Forest are spawned in. This prevents them from despawning completely if the player runs away or is just too far away from the boss to where they could despawn. It will also prevent a premature experience of seeing the boss health bar at the top when the player is inside or near the Dark Tower.