Heroic Characters

Typically a single creature wouldn't have much impact on a massive battle, but if you wish to include your characters into the damage dealt to an army, you can use this variant rule.

Each mass combat round, a character announces what they are doing, they could be a fighter leading the vanguard, or a wizard on the top of a rocky cliff shooting lightning bolts into the enemy.

Martial Characters At the beginning of the phase, they describe their actions and if they are attacking with their weapons, they roll to hit against the army's AC. If they have multiple attacks, they make all of their attacks. Once they roll their attack rolls, and hit, they then roll their normal damage and they count up all the damage they did and then divide that total by 10, rounding normal. That is the total damage they did to the army at large during that round.

Magical Character At the beginning of the phase, they describe their actions and if they are attacking with their cantrips, they roll against the AC of the army or the army must make a saving throw. On a hit, they roll their normal damage and you then divide that damage by 10, rounding normal. That is the total damage they did to the army at large during that round. If they instead wish to use a powerful spell, like fireball, they must be able to cast that spell a number of times equal to the proficiency bonus of the opposing army and they immediately expend that number of spell slots, the spell must have a large area of effect per the DM's discretion. Take the lowest level they expended a spell slot from and they then roll damage as if they cast the spell at that level. If they require an attack roll, they must make the attack roll. If it requires a saving throw, use the army's statistics. Then take that damage and divide the total by 10, rounding normal. That is the total damage they did to the army at large during that 1 round. If they expend additional slots after the initial required spell slots, they can deal additional damage equal to the level of the spell slots divided by two. This number is then added to the total damage before it is divided by 10. They must announce this before they roll damage. Certain spells, like meteor swarm are powerful enough they only have to cast it once instead of expending multiple spell slots, but that is up to DM discretion and the size of an army should be taken into account when determining how much damage is dealt. Multi- and Single-use Features Most single use features aren't useful enough for a large combat that takes hours to complete, for this reason single use features, like the fighter's Action Surge, isn't useful in this context. This might be true for many other features, like the battlemaster's Battle Die, but it is up to the DM's discretion if characters can use these features during the action. It is doubtful a character would be able to take a short rest during mass combat, but that is also up to the DM's discretion.

Missed Attacks If the characters miss on their attack rolls, they only deal half damage on their attack.

Taking Damage and Exhaustion Over the course of a battle, characters are going to suffer damage no matter how high their AC is. If a character is taking part in the battle, the opposing army makes a separate attack roll against the character and deals damage as if they were attacking another army. This does not count against the number of attacks they can make with their action. Due to the strain of battles and that they are fighting for hours on end, at the end of every round, a character must succeed on a Constitution saving throw against a DC 10 or half the damage they took during the round, whichever is higher. On a failed check, they suffer one level of exhaustion.

Improving the Morale A suitably charismatic character can attempt to decrease the Morale DC while an army is resting. Instead of the army making a Charisma (Persuasion or Intimidation) check, the character can attempt this check instead. They must have at least a 13 Charisma and be proficient in the chosen skill, they must also be able to spend several hours as they perform this check. For a small army, they must be able to spend 2 hours, for a medium army 3 hours, and for a large army 5 hours.