Where does Reactive Armor come from in TFT Set 17?

Reactive Armor is a Factory New crate item.

Reactive Armor

Taking damage from enemies grants 4 bonus Armor and Magic Resistances, up to 50.

graves

Taking damage from enemies grants 4 bonus Armor and Magic Resistances, up to 50.

Armor

The physical-defense stat that reduces incoming AD damage. Tank items, the Bastion trait, and champions or items that grant flat Armor all feed into this mechanic, alongside the handful of effects that scale a bonus off a unit's Armor value. Sunder is its inverse — a debuff that reduces enemy Armor.
Magic Resist

The magic-defense stat that reduces incoming AP damage. Many sustain items, the Bastion frontline trait, and "Gain X Armor and Magic Resist" effects grant or scale off it; Shred is its inverse.