This achievement relates to Bard's Tale III: Thief of Fate.
In Skara Brae, after defeating Brilhasti in the Underbrae, you'll be back at the ruined Review Board and the man there will reward you for your success. This also opens the opportunity to change any of your casters into a Chronomancer. This must be done in order to advance the game.
Be warned that you lose all previous spell levels! You can make multiple mages into Chronomancers, from the look of things, but I wasn't willing to change over more than one... that's a lot of spellcasting to lose!