What’s behind the Rift

The rift comes from entities we cannot fathom. They created the rift to release the Shimmer, and then created the Elder Gods as slaves to build a gigantic Forge that swallowed a large portion of Shimmer and concentrated it before sending it to an unknown location.
The Elder Gods were then left as useless, dying creatures. They figured out the power of the Shimmer and concentrated it for themselves, fusing with Cores, thus ensuring their survival. That fusion however affected their minds and they lost memory of that time.
Each of the Elder Gods went on their way and created Dominions, reproducing with humans and other creatures the way they had been treated before.
They somehow incorporated the core design element of the Forge, a circle with three triangles, into their own designs.

What power is the Shimmer

We do not know. What is know is that it follows a logic: Shimmer concentrated becomes Corrosion, and concentrated even further becomes a Core. Cores eventually fragment and shatter when used, thus releasing a massive amount of Corrosion.
All of those forms of the Shimmer affect living beings. They will be affected physically or mentally with new abilities or changes of form. But there will always be a price to pay in using the Shimmer.