The Bethesda Morrowind-onward RPGs had their engine roots in Gamebyro, which was fairly widely used by other developers as well. That said I think there's a que

The Bethesda Morrowind-onward RPGs had their engine roots in Gamebyro, which was fairly widely used by other developers as well. That said I think there's a question about where the value comes from, as their customization to it 25+ years ago and what it enables their game/toolkit to do has clearly given them a 'special sauce' that hasn't been replicated. Also upthread Source was mentioned which has id's Quakeworld/Q2 engine as an ancestor, via HL1 GoldSrc.