The Story of Maokai
Maokai was a young treant of the Blessed Isles — one of the nature spirits that lived in symbiosis with the islands' unique magical ecosystem. When the Ruination swept the Isles, it corrupted everything it touched: the water, the land, the living things. Maokai was submerged in the Ruination's magical backwash and emerged from it as something twisted — his form contorted, his nature darkened, his connection to the life he'd known severed.
What sets Maokai apart from most Shadow Isles entities is that he's fighting it. The corruption warped him, but it didn't consume him completely. He retains enough of his original nature to feel the wrongness of what the Shadow Isles has become, and that revulsion gives him something the other undead lack: a reason to resist. He fights the Black Mist not because he's good, exactly, but because he remembers what the Isles were and cannot accept what they are.
He carries a sapling of the Blessed Isles within himself — a piece of the living magic that existed before the Ruination. He protects it obsessively, because it represents the possibility that the corruption isn't permanent, that healing isn't impossible. He is angry all the time, which is the natural response to watching your entire world die and spending subsequent centuries fighting the thing that killed it. The anger keeps him moving. The sapling keeps him hoping.