Created for Raven Software?
There is conflicting stories regarding the reason for the creation of the engine. Here's what Robert Morgan told me via email:
The engine is based on the engine given [by] id called the Raven engine. Id moonlighted at Softdisk Publishing and this engine was given to them as part of the legal compensation deal. So we licensed it from Softdisk Publishing, not directly from id. I have the original engine source given to Softdisk publishing in addition to the modified version we used in Greed. I made a ton of changes to the engine to support things like transparency and many, many bug fixes. There's a nasty overflow problem in the rendering code in the original engine. The original used a flood fill sort of algorithm to determine visible walls that would infinite loop at times. I fixed some of the sloping floors/ceiling bugs too.
The name of the engine does seem to indicate that the previous belief is more correct, but there does seem to be more to the story in any case. Also I believe the raven engine source code contains an unreleased tech demo game TODD3D. Blzut3 (talk) 00:02, 2 June 2015 (CDT)