Lelouch vi Britannia is the former 11th prince of Britannia. He has an ability known as geass that forces people who see his left eye to obey his commands. They must see his eye as he gives the command for it to work. He can command multiple people at once. He can only command any given person once.
Geass Enhancement Edit
After using geass a lot, it gets more powerful(or uncontrollable) in some way. In Lelouch's case, he became unable to control when it was activated.
Code Geass Edit
After killing Charles, Lelouch obtained Code and Geass. From what I understand, this lets him retain his geass, while essentially gaining immortality. The only way for him to die would be for him to make a contract with someone else, like the one C.C. made with him, and for that person to kill him. He also has geass in both eyes. As far as I can see, this has no usefulness and represents no change in how his geass works.
In a debate, you could choose Lelouch before his geass becomes uncontrollable, after, or possibly after he gets it in both eyes, however he might be considered immortal at that point.