Debuff
Pain Link
The Pain Link debuff causes the affected Champion to receive a percentage of all damage received by the enemy who placed this debuff. The enemy receives 100% of the damage inflicted to them.
The amount of damage received depends on the enemy who placed the debuff. If it was the Head of Suffering, then the Pain Link debuff causes the affected Champion to receive 15% of all damage received. If the Pain Link debuff was placed by a Champion, the damage received will be specified in that Champion's skill description. If it is not specified, it will cause the affected Champion to receive 15% of all damage received by the Champion who placed the debuff.
The Pain Link debuff can be blocked or removed, and can have its duration increased or decreased. Receiving a Pain Link debuff will awaken Champions under a Sleep debuff.
If a Champion is under both a Pain Link debuff and a Block Damage buff, they won't receive any damage from the Pain Link debuff.
A Champion under a Pain Link debuff won't receive any damage from it when the enemy who placed the debuff receives a weak hit.
What the game's own status effects screen says
In the game data this effect appears as MirrorDamage.
Champions that apply it
5 champions have a skill that applies Pain Link, according to the effect lines in the game data. This lists who applies it, not how reliably: chance, duration and resistance are on each champion's page.
The description is the game's own text. It has not been tested in battle here, and the game's wording can be loose (for example about numbers that differ between versions of an effect). See Data quality for what has and has not been checked. Found a mistake? Use "Report a problem" at the bottom of the page.