Emberheal

Name: Emberheal
Level: 2nd level
School: Evocation
Casting Time: Reaction or 1 action
Range: 30 feet
Components: V, S, M (a small piece of coal)
Duration: Reaction Instantaneous, Action Concentration 1 minute
Description:
As a reaction when you or a creature within range takes damage, you can release a burst of flames from your body. Each creature within 5 feet you, must make a Dexterity saving throw. A creature takes fire damage equal to half the amount of damage taken by the triggering creature, or half as much damage on a successful one.
Alternatively, as an action, you can choose a willing creature within range and imbue it with the power of fire. The next time that creature takes damage, it can use its reaction to release a burst of flames from its body. Each creature within 5 feet of the target, must make a Dexterity saving throw. A creature takes fire damage equal to half the amount of damage taken by the triggering creature, or half as much damage on a successful one.
In addition, once the damage has been dealt, the caster may choose one creature within 30 feet of them. That creature regains hit points equal to half the amount of fire damage taken from this spell, rounded down.
At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the fire damage increases by 1/4 more damge for every 2 spell slots above 2nd level.
If a creature takes 20 points of damage from an attack and uses the reaction option of the spell, each creature within 5 feet must make a Dexterity saving throw. If one of the creatures fails the saving throw, it takes 10 points of fire damage (half the amount of damage taken by the triggering creature). If it succeeds, it takes only half as much damage, which would be 5 points of fire damage.

Examples: You will need to make a spreadsheet, like really. It's totally worth it though.
If a player uses the action option of the spell to imbue a willing creature with the power of fire and that creature takes 15 points of damage on its next turn, it can use its reaction to release a burst of flames from its body. Each creature within 5 feet of the target must make a Dexterity saving throw. If a creature fails the saving throw, it takes 7 points of fire damage (half the amount of damage taken by the target creature). If it succeeds, it takes only half as much damage, which would be 3 points of fire damage.
At the end of the player's turn, they can choose one creature within 30 feet of them that took fire damage from this spell. If that creature took 10 points of fire damage, it regains 5 hit points (half the amount of fire damage taken from the spell, rounded down).
If the spell is cast using a spell slot of 4th level or higher, the fire damage increases by 1/4 more damage for every 2 spell slots above 2nd level. For example, if the spell is cast using a 4th level spell slot, the fire damage dealt would be increased by 1/4 (rounded down) of the base damage. If the base damage is 10 points of fire damage, the increased damage would be 2 points, resulting in a total of 12 points of fire damage dealt.