This is no mere vision. Lelouch is physically in the Thought Elevator/Sword of Akasha confronting Charles. He uses his geass and Charles shoots himself through the heart.
But it isn't that easy.
The final stage of geass is to absorb the progenitor's immortality.
Charles has taken V.V.'s immortality.
C.C.'s ultimate goal is for Lelouch to take hers. Or it was, anyway. She arrives to confront Charles and sends Lelouch into her own memories.
But Lelouch will not be removed. After learning C.C.'s history, he returns and destroys the Sword.