The Keystones

The Keystones are believed to be two pieces of ancient design that holds the key to opening a vault in Sersinara.
According to Goddess Laer in Tale of Two Sisters: Episode 07 - The Talking Tree, the keys were created by The First King of Silmeira, to lock away a great beast, one capable of destroying worlds.
One of the keystones is discovered by The Crimson Companions inside a Silmeiran temple in Tale of Two Sisters: Episode 06 - Matching Medallions. It's unclear who had put it there or how long it has been there.
According to Animaera Ilntero in Tale of Two Sisters: Episode 09 - The Travelling Twins, Arseira of The Legion of The Hidden has the other key. Which means both keys are known.

Known Keystone Bearers
Wilhelm Ironbark (Current)
Gnaw'd Rotflesh
Arseira (Current, possibly)
First King of Silmeira (End Times)