Mercenary

Slade Wilson is the best at what he does, and what he does is whatever the person with the most money wants. So bringing hell to Equestria at the behest of a demon? No problem, well, as long as he pays.

Slade Wilson, also known as Deathstroke the Terminator, master mercenary and assassin has been hired to invade the land of Equestria with the intention of assisting a cult in destroying the seal that keeps Trigon from releasing his evil upon the planet.

Easy enough, right?