After you complete the game once on any difficulty you will have the option to play
New Game+ which is the same as starting a new game with the bonus of keeping all the Relics, Talents, and Essence from your last playthrough. You can also use it to level select.
Just choose whatever difficulty you like and play through the game a second time. If you choose this playthrough to play on
"Don" difficulty for the
Don of Darkness achievement, don't be too concerned. The hardest difficulty on this game is actually very easy compared to many other games.
If you ever get stuck for any reason,
click here for a great walkthrough reference for the game and its collectibles.
Good luck!
Edit: A word of caution from Minute Horizons:
Minute Horizons said:When I started my new game plus I selected "From the beginning" (Don difficulty). None of my relics or talent upgrades had been carried over. I played on for a bit to see if they became available after the tutorial sort of bit but they didn't.
Then I returned to the main menu and selected the new game + again but selected the chapter I had played up to and I had all my talents, etc.
Just thought I'd add this since i couldn't find any info on why my talents were unavailable.