Soul Vial

The Soul Vial is an item added by the Ender IO mod. When in the hand it's functionality is very similar to the Reusable Reusable Safari Net in that you can pick up mobs and then drop them down later without consuming the Soul Vial. Combining a Soul Vial that has a mob in it with a Powered Spawner in an Anvil with 30 levels will change the mob type of the spawner. An Enderman vial is also required to make an Ender Crystal in the Soul Binder.

More Info
Soul Vial on the Ender IO Wiki