Buff
Magma Shield
The Champion's HP Bar is reinforced with a Magma Shield for the indicated number of turns. Damage is calculated as normal, but is first applied to the Magma Shield (unless the attack is stated to ignore Shields). When the Magma Shield buff expires, or the value of the Magma Shield reaches 0, the buff is removed and any further damage is applied as normal.
When damage is dealt to a Magma Shield, it is first applied to the Magma Shield, then an equal amount of damage is also dealt back to the attacker.
What the game's own status effects screen says
In the game data this effect appears as MagmaShield.
Champions that apply it
5 champions have a skill that applies Magma Shield, 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.