This card is a rare case where its Materials were changed after being released; in the anime, it requires 2 Level 5 monsters, while in the OCG, it was changed to require 3. This was likely due to its high ATK for a Rank 5 monster.
A "Requiem" is a Mass in the Catholic Church offered for the repose of the soul or souls of one or more deceased persons. This is represented by the energy wings and orbs of "Dark Requiem Xyz Dragon" having a distinct stained glass pattern like those found in the windows of Catholic Churches.
It is also fitting as "Dark Requiem Xyz Dragon" appeared when Yuto was feeling grief for leaving his friends to the mercy of Duel Academy and many of them were turned into cards as a result.
Also, "Rebellion" being replaced with "Requiem" in the name mirrors Yuto being apart of a rebel group fighting the Duel Academy to mourning his friends.