Kill a baby brontosaurus in Hellbent Mountain while unformed and visible.
From recall, go: (sw) 4e n 2e n w 2n 2w n w n 2w d (ne) d 5s u e d 3(ne) u s e (se) 2w 4s 2e n (ne) e d 7w u s d 3(se) u w s w (sw) s w s d 7n u w d 3(sw) u n. They're in that area. (You can't really speedwalk because of aggro mobs.) The highest alignment that has been confirmed is -350, and was confirmed by Rajani. Having a lower alignment does not necessarily speed up the process.
For killing a defenseless, innocent baby, the Powers have granted thee a Mark. You have been granted The Mark of Anger! You gain 38 practices!
This mark also grants -100 alignment.