The Soul Shards mod allows you to create items called Soul Shards. The Soul Shard, after killing an appropriate number of mobs, can then be placed in a Soul Cage for spawning. There are 5 tiers for the Soul Shard. The higher the tier level, the higher the spawn rate. Tiers are gained by killing multiple mobs of the same kind, and higher tiers need a higher number of kills. The Soul Shard gets fixed to the first mob you kill and only acts and adds kills to it's increment if it's in your hotbar. You can also get 200 souls of the mob if used on the appropriate mob spawner by right clicking.
Please be wary of the multiplayer server capabilities when using tier 4 spawners. Left unchecked, they can quickly fill a chunk with thousands of mobs. It is recommended to use tier 1, 2, 3 and 5 spawners only as 1-3 only works while someone is nearby, and tier 5 can be turned off completely with a redstone signal.