A homebrew system for parties who want to earn battle scars.
When a creature of level 3 or higher takes damage equal to half of their hit point maximum from a single source, they must make a DC15 Injury Saving Throw. The major injury table and saving throw ability used are determined by the type of damage received.
Variant. If tanks want to start collecting scars sooner, and squishies want to put it off longer, instead of beginning to risk major injuries starting at level 3, instead begin when each character reaches or surpasses a hit point maximum of 20.
A creature who succeeds takes the damage as normal but suffers no other effects. A creature who fails is considered Injured and must use their Injury Saving Throw result to determine the injury they have acquired from this devastating blow.
A creature who is Injured can be treated with the medicine skill to avoid potential complications that may arise from magical healing (as described on the major injury table). In addition to the effects determined by the major injury table, an injured creature has disadvantage on concentration checks until they receive medicinal treatment and finish a long rest.
Optional: Once a creature has failed an Injury Saving Throw, other creatures cannot receive major injuries for 24 hours (this ensures that the dramatic spotlight doesn’t become diluted and the party doesn’t death spiral during particularly harrowing combats).