Collect 1,000,000 Souls.

This one can be a bit of a grind. By the time you reach Scar, souls clusters will come in the thousands. Hold at least 75,000 souls and go to an area where you can die easily and are close to a serpent hole. I found it best to go to Haven - North End, go to the right as you run into a dagger-throwing demon and a cloaking one, and let them kill you. Keep collecting the floating soul and repeat until the achievement unlocks.

Souls count does carry over to other playthroughs.