When your Darkness powers are active you will often see glowing portals on the ground. The game teaches you how to summon the different Darkling types as you progress through the game, so make sure to always summon a Darkling when you see one of these portals. You should get this achievement with ease very early on in your first playthrough of the game.
Also, for a little more info, there are different Darkling types you'll "unlock" as you progress that help out in different ways. All of them count towards this achievement, but you will probably get it before you even discover the next Darkling type.
Completing this achievement gets you progress towards the following achievement: