Soul Shards is a simple mod that allows you to store the souls of creatures inside gems for use in a Soul Cage to summon more of them later.
In the current releases, you obtain a shard by building a small structure and right clicking the center with a diamond. The first kill you get with a Shard in your inventory binds that shard to the killed entity. It will only receive souls from that entity from then on. To speed up the process, you can right click a Mob Spawner that contains the bound entity to increase it's stored amount by 200 (configurable).