After reaching level 15. a new option will appear in the forbidden forest: Seek Out Voldemort. When you do so, you will find Lord Voldemort, the most powerful enemy. Before you face Voldemort, you will be given the option to run away like a baby; however, if you do so, you will lose a charm point and you will not be able to Seek Out Voldemort for the rest of the day. It is recommended to have all your specialty uses (such as Thieving Skills, Divine Powers, etc.), full hit points, and any other perks you may have, such as mounts or pets, before attempting Voldemort.
If you defeat Voldemort in combat, you will be given the option to continue. If you have under 50 VEs, you willn be given a new title and chance to choose a different specialty use. You are also allowed to choose your race. In addition, you will be given a point you can spend on adding forest fights, adding 5 max hitpoints, or boosting your attack or defense. You will be given 5 charm. Pets, phoenixes (assuming you have the qualifications), any additional max hitpoints you earned, a hat (somewhat reduced), charm, gems, and merit points from the Order of the Phoenix are carried over to the next title. The important thing is that gold is not. You can either blow off your gold on a hat at Deimos' Haberdashery, or you can store it in a dwelling assuming you have the key to one and there is room for the gold. Weapon and armor names change every time (I believe) you VE, but the cost never changes. As you VE, you can unlock certain things in the world such as better mounts and new areas (Ministry of Magic, Spinner's End, No 12 Grimmauld Place). VEs directly correspond to titles, which change every time up until 50 VEs.