Two Tools for Mass Combat– Mobs and the Red Shirt Save

Mass combat can mean one of two things: regular combat with the norma rules in which larger groups of weaker enemies are treated as a single unit (usually called mobs), or larger battles on a larger scale (both in terms of space and time), using a different set of rules to simulate military-style battles.

This article is about the first of those scenarios.  If your players are going to be leading thirty villagers in a fight against fifty bandits or something like that, here's how you do that.  Here's a simple system for running mobs, as well as a special rule you can use to let player characters, BBEG's and other important people benefit from having a squad of mooks under their command

OSR Mob Rules

A bunch of identical, or at least similar combatants can be combined into a mob.  A mob has the following qualities.

• Its hit points are the sum total of the hit points of all its members.  Every time it loses a number of hit points equal to one member, it loses a member.  Or you could interpret this as several people being wounded and the mob loses the equivalent combat effectiveness of one member since it's unrealistic that people would get wounded one at a time.  But the point is, if a mob of ten bandits with 8 HP each loses 8 HP, it's treated mechanically as being down to nine bandits.  
• The space it takes up is equal to at most what its members would take up individually, and at least half that.  So the ten bandits take up five to ten 5x5 dungeon tiles.  Mob members can stand closer together without penalty.  The shape of the mob can be whatever it needs to be.  
• If the mob is hit by an AoE like a fireball, roll damage once and multiply it by however many members would realistically be hit.  
• In larger battles, morale rolls might be made for individual mobs rather than the whole side.  Mobs make morale rolls based on losses in the mob itself– so when they lose a member, a leader, and half their members.  Any morale rolls made due to losses of allies from outside the mob are made with advantage; mob members primarily draw courage from the people immediately around them.  
• When a mob is "killed," that means all members are incapacitated, not necessarily dead. 
• If a mob is in close formation, ranged attacks against it have advantage.  
• If anyone is in melee combat with a mob, the whole mob has disadvantage on ranged attacks, even members not personally in melee contact with the enemy.  

Mobs generally attack as a group when possible.  Instead of rolling dice for mob attacks, cross-reference the number of members in the mob with the raw d20 roll needed to hit to see how many members hit.  If they have advantage, subtract 4 from the number needed to hit; if they have disadvantage, add 4.  

So if ten bandits with +1 to hit attacked a character with AC 16, they'd need to roll a 15, so they'd hit 3 times.  You can roll damage for every hit, but if it's more than 2 or 3 you probably just want to roll once and multiply by the number of hits.  

If it's a zero, roll a d20; on a natural 20 one person hits, but not critically even if you otherwise use critical hits.  

Any enemy inside the mob gets attacked from all sides, with advantage, by as many people can reach them.  Usually that means as many as eight people.

The Red Shirt Save

If a player character, BBEG or some other important person is leading a mob, you might want to simulate the chance that an attack aimed at them hits one of the mooks instead.  The red shirt save is the simplest way to do that.  

Count up how many mooks are close enough to take the hit for the character.  Generally that means they have to be within 5 feet in any direction, or directly between the target and attacker.  Usually there won't be more than 5-10 who qualify unless you have a huge mob and aren't leading from the front.  

When the named character gets "hit" by an attack, roll a d20.  If the number rolled is equal to or lower than the number of mooks close enough to take the hit, one of them gets hit instead.  

Named characters should still attack and act separately from a mob even if they're leading it, since they'll presumably have totally different stats, attack bonuses, AC, damage, etc.  Let any player who's leading a mob control it and roll for it though.