This achievement is for Slaying all 10 Revenants in NG+
The Hellfire, The Maw, The Mauler, The Impaler, The Banshee, The Ravager, The Bile, The Wail, The Hellborn, The Flayer.
The Video below shows the location of all the Revenants in game.
Video Credit goes to=lockongaming
https://www.youtube.com/@lockongamingelite