Tl;dr

• If the minion takes any damage from an attack or the result of a failed throw, their hit points is reduced to 0
• They have hit points and AC
• They can be overkilled,
  • Weapon attack deals 18 damage to a minion with 5 hp, take down 3 total minions
• Group attack
  • 2-5 minions attack with one attack roll
  • +1 attack bonus for each minion in the attack
  • If the attack hits, multiply damage done by the number of minions (most of the time minions only do 1-2 damage)
• Roll one saving throw for each minion or group them into chunks of five

Sample statblock

Written Rules

General

A minion is a weak foe, designed to allow GMs to create dramatic combat encounters with hordes of enemies without overwhelming the characters. In fact, an encounter with minions makes characters feel heroic, since they can take on a myriad of foes and live to tell the tale. However, minions still make threatening foes. Killing a minion still requires penetrating their defenses, and characters can’t just shrug off damage from minion attacks. So how do minions make running a horde of enemies quick and easy for the GM?

• Minions are simple to run. Their stat blocks are small and uncomplicated. 
• Minions act quickly. They don’t multiattack, roll for damage, or take unique bonus actions or reactions, so their turns aren’t long. 
• Minions die fast. A character can kill several minions with a single weapon attack! 
• Minions have strength in numbers. Their attacks can be grouped together to make them deadlier and faster to use at the table.

No Hit Dice 
Minions have hit points but no Hit Dice, simplifying their design. Minions can’t spend Hit Dice to heal during a short rest because they have none.

No Damage Rolls 
Minions don’t roll for damage because their attacks deal a static amount of damage. They also can’t score critical hits.

Shared Turns 
Typically, all minions of the same stat block act on the same turn. Since they share a turn, the minions can each move into position then each use an action if they wish, instead of each moving and taking an action individually.

Minion Trait 
Every minion has the Minion trait, which affects the creature in the following ways: 
• If the minion takes any damage from an attack or as the result of a failed saving throw, their hit points are reduced to 0. 
• If the minion takes damage from another effect, they die if the damage equals or exceeds their hit point maximum; otherwise they take no damage.

Overkill

Overkill Attacks 
Powerful weapon attacks can kill more than one minion in a single maneuver called an overkill attack. 
As already discussed, a weapon attack requires only 1 point of damage to reduce a minion to 0 hit points, regardless of their hit point maximum. However, when a weapon attack’s damage does exceed the target minion’s hit point maximum, the attack becomes an overkill attack and the damage dealt beyond the minion’s hit point maximum becomes overkill damage. 
Overkill damage can be applied to a second minion who has the same stat block as the target and is in overkill range (see below). Damage against the second minion is counted as if you made a weapon attack against them; since it only takes 1 point of weapon damage to reduce a minion to 0 hit points, any amount of overkill damage immediately knocks them out. But wait, it gets better—if the initial attack’s overkill damage exceeds the second minion’s hit point maximum, the leftover overkill damage can roll over to a third minion, and so on! In other words, for each time the overkill damage exceeds the new target’s hit point maximum, the attacker can choose an additional minion to reduce to 0 hit points. 
For example, when a weapon attack deals 18 damage to a minion with a hit point maximum of 5, the overkill damage is 13. If there are three additional minions of the same stat block in overkill range, they can all three be immediately reduced to 0 hit points, since the overkill damage exceeded the target’s hit point maximum more than twice over.

Overkill attacks can’t be made as part of an opportunity attack.

Overkill Range

Melee Overkill Attacks.

When a creature hits a minion with a melee weapon attack, other minions within reach of the attack are in overkill range and can be chosen as additional targets for an overkill attack. The overkill attack can’t target minions outside the weapon attack’s reach.

Lady Ulnock the paladin battles a horde of goblin minions (each with 6 hit points). She hits a goblin minion with her longsword and uses Divine Smite, dealing 8 slashing damage and 11 radiant damage to the target for a total of 19 damage. Since Lady Ulnock dealt 13 points of overkill damage—more than the hit point maximum of two additional minions—she can choose up to three additional goblin minions within 5 feet of her (the reach of her longsword attack) and reduce them to 0 hit points. If there are no other goblin minions within 5 feet of Lady Ulnock, she can’t damage additional minions with this attack.

Ranged Overkill Attacks.

When a creature hits a minion with a ranged weapon attack, other minions in a line originating from the creature in the direction of the target, to a distance equal to the weapon’s short range, are in overkill range and can be chosen as additional targets for an overkill attack. The overkill attack can’t target minions outside the line or beyond the weapon’s short range.

Perigold Quickfingers the rogue is hidden and takes aim at a group of zombie minions (each with 6 hit points) with his light crossbow (which has a short range of 80 feet). He hits a zombie minion with his crossbow, dealing extra damage thanks to his Sneak Attack, for a total of 14 damage. Since Perigold dealt 8 points of overkill damage—more than the hit point maximum of one additional minion—he can choose up to two additional zombie minions in an 80-foot-long line extending from Perigold in the direction of the target, reducing them to 0 hit points. If there are no other zombie minions in the line, then Perigold can’t damage other minions with this attack.

Group Attacks

Each minion has at least one group attack action that speeds up play. In a group attack, two to five minions of the same stat block who share a turn can all use their action to join the attack, provided the target is within the original attack’s reach or range for each minion. 
• Make a single attack roll for the group attack. It counts as one attack. 
• A group attack roll gains a +1 bonus to the attack roll for each minion who joins the attack. (For example, if four goblin minions make a group attack together, the attack roll has a +4 bonus.) 
• If the group attack hits, multiply the damage by the number of minions who joined that group attack. (For example, if four goblin minions hit with a group attack that deals 1 damage, their group attack deals 4 damage.)

The GM decides how many minions join a group attack. For instance, if five minions surround a target, the GM may decide to have all five attack at once to speed up combat, or may break up the attacks among smaller groups to increase the odds that some minions hit while others miss. A single minion can even use their group attack action on their own—they make the attack as a normal creature would, and simply don’t benefit from the group bonuses described above.

Advantage and Disadvantage 
A group attack is only made with advantage or disadvantage if all the minions joining the group attack have advantage or disadvantage on the attack roll. Otherwise, the attack is made without advantage or disadvantage.

Cover and Concealment 
If a target has cover or concealment from some but not all minions, the GM should divide the minions into multiple groups based on the type of cover or concealment they have, then make a separate attack for each group.

Target Response Effects 
If a group attack triggers a reaction or similar effect that would normally affect a single attacker, such as the fire shield or hellish rebuke spell, the target of the group attack picks one minion who joined the attack to be affected by the effect.

Group Opportunity Attacks 
If a creature provokes an opportunity attack from more than one minion of the same stat block at a time and those minions have a melee group attack action, the minions can each use their reaction to join a group attack as an opportunity attack.

Optional Rules

Optional Rule: Group Saving Throws 
Though minions often make saving throws individually, there are times when rolling individual saving throws for each minion could slow down the fight, like when a cleric surrounded by eighteen shade minions uses Turn Undead. 
When many minions with the same stat block need to make a saving throw against the same effect at the same time, you can make one saving throw for a group of up to five minions at a time. All minions in a group use the result of the saving throw.

For instance, if thirty-four goblin minions need to make a saving throw against a hypnotic pattern spell, the minions would make a total of seven saving throws against the spell: six for thirty minions divided into six groups of five, and one more save for the remaining group of four minions.

Optional Rule: Tough Minions 
Minions of a higher challenge rating, such as abyssal ghouls or fire giants, make for powerful foes. Consequently, it could break the game’s verisimilitude for an NPC commoner to kill such a minion with a single attack. To keep minions believable, you can use the following rule.When a minion’s challenge rating is at least 6 higher than an NPC’s challenge rating, that NPC’s actions and traits affect a minion as if they didn’t have the Minion trait, reducing the minion’s hit points like a normal creature instead of automatically dropping them to 0.

This rule shouldn’t be applied to player characters— they’re heroes who can always kill a minion in one hit.

Special Traits

Many minions have traits that give them strength in numbers but become less powerful as their allies are defeated. For example, an enemy who starts their turn within 5 feet of three or more goblin lackeys must succeed on a saving throw or take damage from the lackeys’ Tiny Stabs trait.

Encounter Building

Challenge Ratings 
Minions have a challenge rating just like any other creature. However, their experience point value depends on their challenge rating, as shown on the Minion Encounter Building table. Aside from their damage output, the Minion trait, and their experience point value, a minion’s statistics are on par with a standard creature of the same challenge rating. 
When the rules reference a challenge rating, such as the polymorph spell or the cleric’s Destroy Undead feature, use the minion’s listed challenge rating as normal. For instance, a 5th-level cleric can affect Undead creatures with a challenge rating of 1/2 or lower with their Destroy Undead feature—so this feature can affect rotting zombie minions (CR 1/4) but not shade minions (CR 1).

Building Minion Encounters 
Minions present opportunities for memorable, cinematic encounters. Keep the following tips in mind to achieve maximum fun while planning and running combat encounters with minions.

Encounter Difficulty 
When building encounters against a single foe, the core rules advise matching the characters with a creature whose challenge rating (CR) is close to the average party level. What happens when you add minions to the mix? 
You can use the Minion Encounter Building table to determine how many minions are equivalent to one standard creature of that CR for encounter building. 
You can increase a minion’s challenge rating using the rules in the “Modifying and Making Minions” section.