Old Growth: TFT Set 18 trait
Old Growth is a single-unit trait in TFT Set 18: it belongs to Maokai and is active as soon as Maokai is on the board.
What Old Growth does
Whenever an enemy within 3 hexes dies, your strongest Old Growth champion gains 30 permanent max Health. Current: 0
From Riot's game data, with the latest patch notes applied; the exact numbers can change between patches.
Maokai