Soul shards

Apparently a major aspect of the world, the shards aren't fully explained. We know they're a major source of power, but different religions interpret them in different ways. Even Riala isn't very certain about them, only saying, "It really is a piece of a soul, or at least similar to one. Whose, I couldn't tell you."

When taken, the shards merge with someone's soul and grant them power, including sexual dominance. Riala says she couldn't use the one she found, but it's uncertain why. Given that all the holders seen so far or men, and that people are allegedly curious about Esthera, it seems likely that shards cannot be taken by anyone, possibly women or succubi. Under normal rules, anyway.

Known Shards
Incubus King

Simon - stated to be pretty small

Esthera - how she got/holds hers seems to be a mystery of some kind

Lustlord

Doomed King - Riala says the armor was forged from a small fragment

Potential Shard Holders

 * Someone in Erosia - Riala implies he's unpleasant
 * King Alonon - seems pretty likely based on his interludes
 * Everyone in the Prologue - suspect everyone >.> <.<

Five Major Pieces
Not all pieces are the same size. Simon's is relatively small and Riala implies there are five that are substantially larger than the others. Exactly who possesses these five is still unknown but some of the holders above are likely among the five.
 * 1) Incubus King
 * 2) Someone in Erosia