Horus Versus Corax
Scenario 1. Legion vs legion in a galaxy wide war utilizing all aspects, such as resources, supply lines, forge planets, human soldier recruits.
Scenario 2. Legion vs legion in a planet wide war
Scenario 3. 10 elite solders of lunar wolves vs 10 elite soldiers of ravenguard in an arena setting, each team starts on opposite sides of the colloseum.
Scenario 4. Horus Versus Corax in one on one combat to the death. Each primarch gets their custom wargear of preference, as well as being able to utilize all their powers, skills, and abilitties to the best of their potential.
Note - characters are fighting for the kill/win to the best of their abilities, but still retain their unique personality traits which give them their strengths, tactics, etc.