Ultima VII

Ultima VII

In the seventh installment of the Ultima series you — the Avatar — must go back to Britannia and investigate a series of strange events. Two hundred years have passed since your last visit and much has changed since then. An unknown being who calls himself the Guardian has appeared and is forcing everyone to bow to his whims. In addition magic doesn't work as it should, it seems that all of the mages have gone insane, and quiet towns are being terrorized with ritualized murders. It is unclear what is happening, but initial signs point to some kind of organization called the Fellowship. It is up to you to unravel the mystery and find out what is really going on in Britannia.

Minimum system requirements: 

The Ultima VII: The Black Gate or Serpent Isle data files and the Exult engine.

External links:
Developer: Origin Systems


free, non commercial, opensource?

Just for clarification: This game is tagged commercial and closed source, because although Exult itself is free, non commercial and opensource, You need to own "Ultima 7: The Black Gate" and/or "Ultima 7: Serpent Isle" (Exult documentation).