The Shadow Servant is the starting weapon of Genevieve Gruyere, a new character who is unlocked after breaking all the seals in the Abyss Foscari level.
In order to evolve it into the Ophion, you must level it up to its max level of Eight while also leveling up a Skull O'Maniac item to its max level of Five. Once both items are maxed out, keep opening treasure chests by defeating mini boss enemies until you get one that evolves into an Ophion.
A great way to get a jump on this is start a map with Arcanas on and select the card VIII Mad Groove. For some maps this will put a Skull O'Maniac (or two) right next to you when you begin the level.
After you get the Ophion, you can either finish the level, fight until you die, or quit immediately. The achievement unlocks during the end level results screen.