What it does
Light of Ruin deals Fairy-type special damage and inflicts recoil on the user equal to roughly half the damage dealt. The higher your damage output, the more HP you lose from recoil.
There's no recharge turn — unlike the Hyper Beam family, you can act immediately the turn after. The drawback is purely about HP. Against Steel-type and Poison-type targets, Fairy damage is halved, which also means smaller recoil, but the efficiency drops hard.
The move's power is very high for a Fairy special move, which is what makes Floette's damage output viable despite its modest Special Attack. The math favors it against targets that are weak to Fairy — Dragon-types, Fighting-types, Dark-types — where even a not-especially-high Special Attack can threaten large chunks of HP.
Who runs it
Floette (Eternal Flower form) is the exclusive user in Champions, at 95% usage rate. That number tells you almost everything: this move is the reason you're running Floette. Its entire offensive identity is built around delivering this one high-power Fairy hit.
How to use it
In doubles, time your Light of Ruin when Floette's HP is healthy and the target is Fairy-weak. Dragon-types and Fighting-types are ideal — you can threaten serious damage in one hit, forcing the opponent to address it immediately.
Recoil means Floette can't spam this freely. If you fire it twice into the same game, your HP will be critically low. Plan for what it does after the first big hit: whether it stays in to finish the job or pivots out depends on how much HP the recoil leaves.
Tips & strategy
Don't click this against Steel or Poison targets — the reduced damage means you're taking recoil for minimal payoff. Against resistant types, Floette's other options are better uses of that turn.
Sitrus Berry is worth considering as the held item — it triggers when HP drops to half, which recoil will naturally push you toward, buying another turn of activity. Watch the HP threshold carefully: if recoil leaves you at low HP, any priority move or spread attack from the opponent can finish you before you get another action.