Where does Latent Explosion come from in TFT Set 17?

Latent Explosion is a Factory New crate item.

Latent Explosion

Store 15% of damage dealt to enemies.

graves

Store 15% of damage dealt to enemies. On takedown, enemies release the stored damage as physical damage in a two-hex radius.

On Takedown

Effect fires when a unit secures a kill or assist. In Set 17, Kai'Sa's passive "On takedown, gain 10 mana" is a clean example, and Seeker's Armguard stacks +20 Armor / MR / AP per takedown.
Damage Storage

Stores a portion of damage dealt or taken during combat, then releases it as a single payload when a trigger fires (on-death, on-takedown, timer, or duration end). In Set 17, Annihilator stores 20% of damage dealt and unleashes it as true damage on death or after a timer; the Anima Squad Latent Explosion bonus stores 15% of damage dealt and detonates on takedown; Aurora's ult marks enemies who store 10% of damage they take and pay it back as true damage when the Hack ends.