Looking for a game engine - Dark Messiah-like game set in Ancient Egypt Thread

I'm planning on making a game that uses mechanics similar to older fantasy games like Dark Messiah of Might and Magic, focusing more on combat and storytelling rather than complex loot tables and stats, and something with the simplicity in design similar to modern FPS titles rather than an open world RPG. I'd really like to make this game a reality, but I'm having difficulty deciding on an engine to build it on; I'm tied between Source, since I can turn the early game into a Source mod, or Unity, since Unity's got a wide range of possibilities and has the advantage over Source in that it's currently being updated and worked on regularly whereas Source is largely abandoned while Valve works on Source 2.

The game will take place in a mythological replica of our own world, the setting in particular being Ancient Egypt. You play as a Hittite slave brought to Egypt initially to join the fighting pits, but after an encounter with a terrifying extremist cult dedicated to Set, you are hired by the prince of Egypt to seek the cult out and determine their threat to Egypt in order to help stop them; your reward being your freedom. The loot of the game will be limited and widely varied and set to different fighting styles, while the combat itself will be fluid and similar in scope to Dark Messiah's. Many of the enemies you will fight will have roots in various mythological cycles, from Ancient Egypt to nearby Minoa, Ancient Anatolia, Babylon, and Nubia. The game will focus on a primarily linear storyline in various parts of Ancient Egypt, with different chapters jumping to different regions of Egypt (some of the places you will visit will include Memphis, Faiyum, Thebes, and the Valley of the Kings).

As I said, I really want to make this game a reality but need to pick an engine that I feel will work for it. I know that I'm willing to use an older engine but I want to make sure that the mapping program won't be too difficult to accomplish for the game itself, so I already know that I don't want to make it for Unreal 4 (whenever I set up the team I hope to be the one in charge of level design as well as story, so I'm also looking to teach myself whichever level editor comes with the engine; I.E. Hammer for Source).

Any help is appreciated. If you have questions about the game I'll be able to answer as best as I can, as I have quite a bit planned for it including what kind of enemies the player will encounter.