Black Rider

Whenever an opponent uses an activated ability
of a creature for the second time each turn, create a 2/2 Black Rider creature token
with menace. If you control 9 creatures called "Black Rider," search your library for a legendary creature spell and put it on to the battlefield under your control. Sacrifice all Black Rider creature tokens.

"There are others. I know their number. I know these riders." Aragorn, Tolkien 206.