Soul Cage

The Soul Cage is a special block that can spawn the type of mob a Soul Shard has absorbed, usually at much better rates than a vanilla monster spawner. A Soul Cage is crafted by using 8 iron bars in a square (Similar to a Chest). You place the Soul Cage on the ground and then right-click your Soul Shard onto the cage. Mobs will spawn until you remove the Shard. The cage responds to redstone if a tier 5 shard is placed in it.

The spawn area is the same as a vanilla spawner. That is, an 8x8x3 area around the spawner (with the odd sides being towards the lower x and z coordinates).

Destroying the soul cage with a pickaxe will drop the soul cage and the last soul shard placed in it, rounded down to the minimum number of souls for its tier.

Here is a video demonstrating the creation of a very highly-productive XP farm using a Soul Cage:



Bugs

 * Destroying a soul cage with a Wither Skeleton shard in it will drop a Skeleton shard of the same tier. Possibly related is the fact that if you place a Wither Skeleton shard in a cage, it will show a small Skeleton inside, rather than a Wither Skeleton, although it will spawn Wither Skeletons.