I'm slowly working through spells here, but Karmic Retribution stood out to me.
it's not a good spell. It could either swing an encounter wildly but has equal potential to fail spectacularly, and it's a very high level slot for not a good spell. This is what i call a "only villains do that" spell b/c PCs would never want to spend a spell slot on this but a villain with nothing to lose would love it. HOWEVER the concept itself is good and just needs some retooling.

you don't specify targeting on the spell other than 60ft range. Effects like Hellish Rebuke that are contingent on getting attacked but only affect one creature are generally REACTIONS, so if you want to make this a stored-damage spell that affects only 1 attacker, make it a reaction to trigger the damage. However, I think 60ft range is too generous. contingent effects, including damage triggered by special effects, are more often Abjurations. Spells that have an explicit HP cost are almost always Necromancies. Radiant damage is associated with justice, not vengeance, and if you're a divine caster casting this spell i'd expect it to do force or necrotic damage.
====suggested edit====
Spell Name: Karmic Retribution
Classes: Cleric, Paladin, [Warlock suggested]
Spell type: v,s
Spell level: 4th level Abjuration [or Necromancy if necrotic is used]
range: self, or 25ft Cube
time: 1 action
duration: concentration, up to 1 minute
components: n/a
description: Any damage taken by the caster while concentrating on Karmic Retribution fuels the power of this spell. The caster must still concentrate on the spell as normal while taking damage. At any point on the casters turn, the caster may end concentration on Karmic Retribution to release the stored damage upon the target, dealing an amount of radiant [necrotic or force suggested] damage equal to the total damage they have sustained while concentrating on Karmic Retribution in a 25ft cube centered on the caster. Targets may make a dexterity save for half damage. in the If the caster is healed while concentrating on Karmic Retribution, the spell damage is not reduced. If the caster's concentration ends involuntarily, the stored damage is released immediately.
====design philosophy====
=10 minutes concentration is too much damage potential for a 1to1 damage trade. a party with a wand of cure wounds and cantrips could stack damage on the other side of a door before ambushing the final boss and nuking him to hell
=1 minute concentration means the caster must wade into the thick of things to make the most of it
=a 25ft cube means, essentially "anything within 2 squares of the caster" which means the caster must be in melee ranged to use this spell well, which fits well on the Cleric/Paladin List.
=Changing the spell so that triggers when concentration ends means that it's less of an all-or-nothing spell. However, losing concentration at the wrong time could result in some pretty disastrous friendly fire, esp if you've stacked a lot of damage.

I'd originally intended it for the more...dark variety of paladin, the ones who're intent on fucking shit up, think of it kinda like the Darkness spell from final fantasy games, where the damage is taken is converted to damage dealt
I can accept that, can shift it so its like a direct attack instead, like all the pain goes into a single strike of a sword or some such.
I can accept the radiant versus another. I'd say force wouldn't be a bad option but thats exclusively a magery damage type, and necrotic is, as far as I've been taught, its like....pure deathly energy.

I'd argue against the warlock, they tend to be super dang squishy, I don't think it'd be worthwhile to allow them that spell
I'd agree with the 1 minute concentration instead of ten, though some of the longer battles might make it a bit...not great I guess? I dunno

1 minute is a long ass time in dnd to be fair. that's 10 entire turns that you need to be taking damage to charge the spell, which means you have to succeed on a concentration check for 10 individual turns with (aprox) a 40% chance of succeeding each time, with the chance of just getting entirely dunked on if you get hit by a fireball because concentration DCS are either 15 con save or 1/2 the damage dealt, whichever is higher.

if you want a single target version, change the range to 30ft, casting time as a reaction, and require either a save for half or a spell attack roll. note that even in the AOE version, casters are exempt from the effects of their own cubes centered on them (like thunderwave).

Clerics have quite a few force spells, and have had them for several editions: spiritual weapon for example, so it's not out of character. in 5th edition, effects that have HP costs almost always deal necrotic, which is the pure antithesis of life, which makes sense if you're turning your own wounds into harming others.

Also, warlock has quite a few spells that proc on being hit: armor of agythys, hellish rebuke. they have a d8 hit die (same as a cleric), proficiency in con saves, false life at will (if you take that invocation), and bonus hit points if you take Pact of the Blade or Hexblade. Warlocks can actually make some pretty potent spellswords if you know how to build them, and this spell feels like it fits in the warlock's portfolio of Hex-like and other malignant spells.

====suggested edit====
Spell Name: Karmic Retribution
Classes: Cleric, Paladin, [Warlock suggested]
Spell type: v,s
Spell level: 4th level Abjuration [or Necromancy if necrotic is used]
range: self, or 30ft
time: 1 action, or reaction
duration: concentration, up to 1 minute
components: n/a
description: Any damage taken by the caster while concentrating on Karmic Retribution fuels the power of this spell. The caster must still concentrate on the spell as normal while taking damage. As a reaction, the caster may end concentration on Karmic Retribution to release the stored damage upon a target within 30ft that has dealt damage to the caster within the last minute, dealing an amount of radiant [necrotic or force suggested] damage equal to the total damage they have sustained while concentrating on Karmic Retribution. Targets may make a dexterity save for half damage. in the If the caster is healed while concentrating on Karmic Retribution, the spell damage is not reduced. If the caster's concentration ends involuntarily, the stored damage dissipates harmlessly.

Fair enough, I'm just used to shit tons of enemies focused on shit tons of pc's XD

I could do it like the Chaostra's Sunder, where its a sweep of negative energy (...damnit thats necromancy xD) at a target

True, haven't had much reason to look through clerics stuff (excpet when compiling a spell list for the chaostra)

Fair, I could extend it to that as well