lichmaster
Hero
I think the interaction with PCs could benefit from more tweaking.Barebones simplified mass combat:
Take a creature from the MM and apply the Mass Combat template.
HP becomes "Morale". Creature loses the ability to roll for damage and always takes the average. Unit gains a "Passive Attack Roll", cannot roll natural 1s on attack and cannot roll lower than the passive attack roll which is 8+Attack mod, unless they have disadvantage. Unit gains a combat role (Artillery, Flanker, Front Line, Support). Unit gets assigned a size.
Small units are 20 medium sized or small creatures or 5 large creatures. Medium units are 50 medium or 10 large, large units are 100 medium or 25 large.
You cannot make units out of creatures bigger than large or smaller than small.
Units take half damage from PCs and NPCs, full damage from Units.
Units deal double damage to PCs and NPCs, normal damage to Units.
When a large unit hits Bloodied it must make a Death Save or become a medium unit.
When a medium unit hits Bloodied it must make a Death Save or become a small unit.
When a small unit's Morale becomes bloodied they must make a Death Save or be Routed/Defeated.
At 0hp a unit is destroyed. There may be a handful of survivors, but they flee the battle or get captured.
A unit cannot be 'healed' into a higher size category. A large unit that fails it's save remains a medium sized unit after the battle.
When a unit increases from small to medium, or medium to large, it gains +50% HP and Damage, and a +2 to Proficiency bonus.
A large unit of 100 bandits is waaay weaker than 100 bandits. Even very high level melee characters would probably die fighting 100 bandits due to 5e's flat math approach, but a large unit of bandits could be defeated quite easily by a level 5 fighter.