Ride Down is an uncommon from Khans of Tarkir. So, in Khans, there are five tri-enemy-coloured khanates, the w/b/r one being Mardu, based of the Mongolian nomads who took over China and swept large parts of Europe. Their theme was wanting to attack a lot, so we have this w/r gold card which encourages you to attack a lot. Want a blocker gone? No problem! Also, in case you didn’t know how a trample creature which is blocked but has no creatures blocking it (ain’t Magic fun?) works, it just does all its damage to the player it’s attacking. I was surprised to find this reprinted in Eldritch Moon, but I guess it does do what many w/r decks would want!