Abrax is a Half-Crazy alchemist that hides in a secret lab under the monastery, he is one of the masters of the monastery and he gives you a quest of find his other novices.
He wants you to find two novices at Tilda's farm, where Severin is in command, and bring back som plants.
Abrax also sells you some of his potions if needed.
You can learn alchemy from Abrax, and he doesn't seem to notice if you plunder his Lab.
In order to get to Master Abrax you need to go down to the secret library and find the way in through a small crack in a wall, you will need to use the nautilus spell to get in there. but you can easily get out again, just pull the lever and a door will open to the lower sleeping quarters.