An evil of the Ancient World, both a shadow and a flame.

The Balrog, Flame of Udûn

Trample
When a legendary creature an opponent controls dies, put The Balrog on the bottom of its owner's library.

Tales of Middle-earth Deluxe Commander Kit

The Lord of the Rings: Tales of Middle-earth

Trample
When a legendary creature an opponent controls dies, put The Balrog, Flame of Udûn on the bottom of its owner's library.

The Balrog, Flame of Udûn will be put on the bottom of its owner's library only if it's still on the battlefield when the ability resolves. If it leaves the battlefield before that point, it will stay in whatever zone it's in, even if it's returned to the battlefield before the ability resolves.