The "most Arby's sandwich Arby's has ever made" features roast beef, cheddar cheese sauce, curly fries, Arby's sauce and Horsey sauce on a toasted sesame seed bun.
The Arbynator was first introduced in November of last year (I tried it back then). It basically takes elements of Arby's Roast Beef, Beef 'N Cheddar, and Curly Fries and puts them together along with two of Arby's signature sauces to make one sandwich.
Photo via Arby's.