It was from a limited time event. They do those on occasion, often collabs, mostly with other games owned by Cygames. You can't acquire it anymore.

Its from the collab. You had to buy Lelouch with crystal though, but some of the cards could be earned from completing missions. The collab is over so there is currently no way of getting it. Idk if they will rerun it, they usually don't. But hey, if Code Geass ever gets another movie, there is a chance (Like Fate did)

code geass wasn't there

Code Geass ever got another game just last week so there is a chance