You will receive this item in The Ivory Citadel dungeon. You will find it shortly after traversing through the first parts of the level, after the section where you must Soul Split to drop your statue self down a tunnel. You will then revert back, ascend and can use Deathgrip to grab the bomb and destroy the corruption growth blocking your path.

Do so then cross the gap, enter the small room and the Voidwalker will be waiting inside the chest. After you receive it, you will receive the achievement.