Lev the Leviathan is a giant Leviathan that all Sirens fear. She overthrew the Sea god, Poseidon, thus making her the goddess of the seas.
Lev was first mentioned by Jaws (one of her servants) when Thomas and Twilight finally defeated him (or so they thought). He mentioned how she gave him the ability to be reborn. "You can kill me as many times as you want, but you'll truly destroy me. I am your greatest nightmare," he said to them.
She appeared in the epilogue of Code Red's Adventures of My Little Pony: Equestria Girls - Friendship Games, as she was seen killing MetalSeadramon in her hunt for the Evil Schemers.
She only appears near the end of Age of Darkness, where her eye opens and she roars.