Soul Shards

Soul Shards are pieces of the Soul Gem, taken by the Vespiri and smoothed into oval shapes for use of the Temple Tenders.

This allows Enivi who are still learning to sooth and guide the souls while they build up their own magic.
The magic from the Soul Shard can "rub off" on the Vespiri over time, granting them the ability to work without it. This is a sign that a Temple Tender is ready to become a Soul Binder - a Vespiri who travels the land to collect souls and bring them home to the temple.

When the Enivi has graduated to Soul Binder and no longer requires their Soul Shard, it's passed on to another apprentice Temple Tender, and the cycle continues.