UPDATE 2/12/19: The developers have released a patch that appears to of fixed most, if not all of the achievements. Please report and leave a comment if any of the achievements still seem to not work!
UPDATE: The recent patch has glitched some of the achievements for the game and as mentioned by Whisperin Clown: "I won this [achievement] randomly after opening my first chest in a game. Didn't have a single legendary item."
You must acquire 2 legendary weapons, a legendary ability, and a legendary movement ability. You can acquire all through any means such as using a forge, chasing golden chests, loot goblins, and eliminating other players.
You can find legendaries in regular chests however, it is very rare so your best bet would be using the previously mentioned methods.
*NOTE* The four legendary items/weapons must actively be equipped for the achievement and not deconstructed/disenchanted.