Soul Shards: The Old Ways Mod for Minecraft 1.9/1.8.9 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.
Download link: Soul Shards: The Old Ways Mod for Minecraft 1.9/1.8.9