What are some alternatives?
When comparing Tombstone Engine and Unity, you can also consider the following products
Armory3D - Armory 3D is a free and open source game development software developed using Haxe and Kha and made to run inside Blender.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
Panda3D - Panda3D is a video game development engine and rendering software. It is developed and maintained by the Carnegie Mellon University Entertainment Technology Center.
GameMaker - "With GameMaker, I went from no programming knowledge to full-time developer in 2 years!" Seth Coster - Butterscotch Shenanigans - Crashlands. "GMS 2 is absolutely amazing and I cannot get over how great this software is to use".