You must have the Dark Urge to get this achievement.
All aspects of achieving this involve spoilers. Read at your discretion.
*** Spoiler - click to reveal *** Here are the steps taken to achieve this achievement.
- Talked to Alfira in the Grove, leading to her joining her camp. Agree to let her stay and take a long rest.
- During the long rest, you will murder Alfira due to your Dark Urge
- Continue on your merry way, and eventually, after enough long rests, you will get the order to murder Isobel.
- Kill Isobel to achieve a beastly form.
- Keep on chugging until Baldur's Gate.
- Once at Baldur's Gate, go to Bhaal's temple and defeat your lovely murderous sister.
- Continue the game as the Chosen One of Bhaal, and upon the final choice of the game, kill The Emperor and sit upon your bloody throne.
- Tada! Achievement unlocked!