Imagine being able to command obedience at will, everyday life would be an interesting Simon says game. However, for Lelouch vi Britannia he doesn’t have to imagine that life after receiving his Geass. Which begs the question, how did Lelouch get his?
Lelouch got his Geass after rescuing C.C., an immortal woman who had the ability to grant a Geass, the power of kings. The Geass that Lelouch received manifested itself as the “Power of Absolute Obedience”. This ability allows him to place commands in a person’s mind upon direct eye contact.
His Geass sigil is visually manifested in his left eye. He also has the ability to toggle his Geass at will.

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Throughout the series, Lelouch uses his Geass to aid the Order of the Black Knights, a vigilante group fighting against the Holy Britannian Empire. Lelouch himself created and led the Order of the Black Knight through his masked persona, Zero.
Lelouch Geass Weaknesses
Lelouch has used his Geass multiple times to get out of sticky situations and win battles against the Empire. Most notable of which was using it on his best friend Suzaku, commanding the Suzaku “to live on” allowing Suzaku to always survive an encounter.
However, his gift is not without consequences. During a discussion with Euphemia about utilizing her Special Administrative Zone of Japan, Lelouch, while explaining the extent of his ability accidentally ordered Euphemia to kill the Japanese.

She had resisted the command at first and Lelouch realizing his mistake goes after her as she executed the command, resulting in the Special Administrative Zone Massacre. Unfortunately, Lelouch had no other way but to kill Euphemia to stop her as his Geass cannot be used on the same person twice. The incident became his burden, one that he was unable to forgive himself for.