Injuries are an "effect" over "duration." They trigger on:

Getting Crit: (hit scores a critical hit) Massive Damage: (takes 1/2 total health in damage) Goes Down: (is at 0 hit points after taking damage)

Step 1: Injury Effect The Injuries effect is determined by # of conditions fulfilled above, +1 if the creature was already bloodied (this total is called Effect Severity). DM decides the injury location. Injuries have a chance to be especially damaging. Roll a number of target's hit die = Effect Severity. If any are a 1, you reroll that die and roll an additional die, until none are 1. For each such die, increase the Effect Severity by 1.

Step 2: Injury Duration The injury duration is equal to 1 + the number of hit die you rolled a 1 on. The target makes a Recovery save depending on the Duration. Duration (#) Target makes Recovery Save: 1 at the end of each turn 2 after each minute out of combat 3 at the end of each long rest Recovery save: On a successful Constitution saving throw with a DC equal to 15 + Injury Severity, an injury Recovers, reducing the Effect by 1d4 and increasing the Duration by 1. If the Duration > 3, the target recovers only if they succeed on consecutive saves (2 for Duration = 5, etc.) at the end of each week of downtime. Injuries can also be cured by spell healing of spell level greater than Effect Severity + Duration.

Example Injuries guidelines These injury rules are more examples of the how severe injuries should be for any particular tier. You can use these as a reference point for how improvising your own injury effects. For example, if someone gains an injury from being struck by lightning, you can use any of the head injury effects. Use your judgement when picking an injury (e.g. choosing a less severe injury when the more severe one doesn't make sense). Head injuries: Severity tier 1: You have disadvantage on Constitution Saving throws to maintain concentration on a spell when you take damage.

Severity tier 2: You can't take reactions.

Severity tier 4: You can't concentrate.

Severity tier 5: You are stunned. Armor or carapace injuries: Severity tier 1: Your AC is reduced by 1.

Severity tier 4: Your AC is 10 plus your dexterity modifier.

Severity tier 5: Your AC is at most 10. Leg/wing injuries: Severity tier 1: You are unable to take the dodge, dash or disengage actions.

Severity tier 2: You aren't able to increase your elevation when flying

Severity tier 2: Your speed is reduced by 10 feet.

Severity tier 3: Your speed is reduced by half

Severity tier 4: You lose your climbing, swimming, or flying speed.

Severity tier 4: You cannot stand. Arm injuries: Severity tier 1: You have disadvantage on dexterity checks that use your hand.

Severity tier 2: You have disadvantage on your first attack each turn.

Severity tier 3: Regardless of your Abilities or Magic Items, you can't make more than one melee or ranged Attack during your turn.

Severity tier 4: You have disadvantage on all attacks with that arm.

Severity tier 5: You are unable to use that arm. Eye injuries Severity tier 1: You have disadvantage on sight based wisdom (Perception) checks.

Severity tier 2: Creatures that are farther than 60 feet from you are heavily obscured.

Severity tier 4: You are blinded. General injuries Severity tier 2: You have disadvantage on all ability checks

Severity tier 3: You are considered poisoned.