Visit the priest in Ithrilis' Sacrarium and give him a tithe of 100,000 gold.
Note this now does nothing if you are in 2nd/3rd/4th class, so buyer beware!
For giving to the mortal manifestation of the Powers, they have granted thee a Mark. You have been granted the Mark of Tithing! You gain 567,000 experience points! You gain 45 practices!