truesteamachievements.com forgot to update the description
In-game (since v 3.17) it is
"Have three Voidstones socketed in your Atlas.."
This can only be done by killing 3 out of 4 end game bosses: Eater of Worlds, Searing Exarch, Uber Elder, Maven to obtain the Voidstones, and then socket it to the atlas