Lonely Adventurer came up with what I thought was an ingenious idea, a better method of adjudicating “aid another”-type actions to make combat tactics more interesting. I helped him refine it a bit, and then got inspired to spin my take on it up into a full lightweight system, below.
Ability Scores and Checks
Each character has the expected six ability scores rated from 3 to 18. Ability checks are made by rolling a d20, adding your ability score, and trying to exceed (not equal) 10 plus the opposing ability score, or 20 unopposed. Ability bonuses, used to modify hit points and damage, are 1 per 3 points above 9.
Actions and Combat
Each side in a combat acts together, alternating turns. Any side that has the advantage of surprise takes the first turn; then initiative is rolled with a d20 per side; each character from the side that lost initiative whose Dexterity is better than the best Dexterity on the winning side plus the margin of lost initiative acts; each character on the winning side acts; and then initiative alternates from there out. Any character that hasn’t taken an action yet is Vulnerable.
Each character has three actions available: setup, shield, or strike. Setups are resolved first; choose a striking character and targeted enemy, and make an opposed ability check against that enemy, with the abilities used depending on the details of the specific action; for example, tangling a foe with a rope might be an opposed Dexterity check. A successful check gives the striker +1 to hit and +1d6 damage against that target this turn. Next, all strikes are resolved; an opposed ability check is made to hit (often Strength vs Dexterity), and on a success the striker deals their weapon damage to the target; however, a character who strikes becomes Vulnerable until their next turn. Strikes must target a Vulnerable enemy if one exists; by throwing yourself into the fray, you invite retaliation. Finally, if a character chooses to shield a striker, they can interfere with any retaliation, giving +1 to defense, and transferring 1d6 damage from any incoming attack to themself (applying armor).
A character reduced to 0 hit points is incapacitated; they must make an unopposed Constitution check each turn until provided first aid, or die.
The number of characters that can attack a single target, and the number that can attack at all, are limited by space and size. In a corridor, a total of three human-sized characters can attack, split across targets as they choose. In an open space with a front of battle, three human-sized attackers can attack each human-sized Vulnerable target. In an open space where attackers can surround their target, six can all attack one target.
If one side fires ranged weapons as the other closes to melee, the side with ranged weapons can make one free attack per 60 feet of distance, up to a maximum of four, or half as many with heavy weapons; thrown light weapons always only allow one attack on the approach.
Basic Statistics: 1d10 plus Constitution bonus hit points per level, all weapons, armor, and shields
Cleave: When you strike, make an attack against every Vulnerable enemy you can reach; usually no more than three, unless polearms are involved.
Bend Bars, Lift Gates: With a brief surge of adrenaline, you can perform impossible physical feats. A successful unopposed Strength check allows you to bend steel bars, raise portcullises by hand, and shatter thin stone.
Hold the Line: When you shield an ally, you can choose to redirect 1d10 damage instead of 1d6.
Basic Statistics: 1d6 plus Constitution bonus hit points per level, light armor, light or medium weapons, no shields
Backstab: When you strike, add +1 additional damage for each ally who successfully set you up.
Tradecraft: You are 10% likely per level to notice immediately before taking an action that would trigger a trap. When you break into a locked room, you are 10% likely per level to do so quietly enough to preserve the element of surprise. When you pilfer an item from plain sight, it is 10% likely per level to go unnoticed.
Parkour: When you perform acrobatic stunts—climbing walls, leaping onto chandeliers, and so forth—you can take a setup action to benefit your own next round’s attack, regardless of target, by making an unopposed ability check.
Basic Statistics: 1d4 plus Constitution bonus hit points per level, no armor, no shields, simple light weapons only
Cheap Tricks: When you setup an ally with magic spells or with conjurer’s tools like flash powder, give them an additional +1 to hit.
Arcane Blast: You can use magic as a weapon, using your Intelligence to attack and dealing 1d6 damage adjusted by your Intelligence bonus.
Ritual Casting: By taking 10 minutes to perform a ritual, you can solve general problems with magic. Rituals can translate ancient texts, destroy sealed doors, dispel magical traps, move heavy objects, and the like.
Basic Statistics: 1d8 plus Constitution bonus hit points per level, all armor and shields, simple weapons only
Divine Protection: You can shield any number of striking allies with a single action.
Lay On Hands: Once per day per level, you can heal an ally for 1d8 hit points plus your Wisdom bonus.
Turn Undead: You can strike the undead with holy power instead of a weapon; attack with your Wisdom against their Charisma. If you hit, instead of dealing damage, they flee in terror. If your level + the number of times your attack was set up equals or exceeds their hit points, instead they’re burned to ash and dust.
Armor protects a character from incoming damage; light armor stops 1 point of damage, heavy 2 points, and a shield 1 additional point.
Weapons deal damage based on their type and quality. Light weapons deal 1d4 damage if simple, 1d6 if advanced, and can be both used in melee or thrown for use at range. Medium weapons deal 1d6 if simple, 1d8 if advanced; they are either melee or ranged only, and ranged medium weapons require both hands. Heavy weapons deal 1d8 if simple, 1d10 if advanced; they always require both hands, and ranged heavy weapons cannot be used to strike two turns in a row.
Uncover a Great Treasure and survive, and you gain your second level, roll another hit die, and distribute four points among your ability scores. Two more Great Treasures, and you become third level, with the same effects. If a third level character finds a further three Great Treasures, they can retire as a Living Legend.