Following his possession (and subsequent corruption) by the Millennium Ring, Diva takes on a horrifyingly grotesque demonic appearance: his skin becomes dark greenish-gray with numerous cube-like structures protruding all over his body (there are two different versions: the larger, more prominent ones [which are a sickly pinkish-purple color and feature black vein-like cracks] cover most of his body, as well as forms a rigid cube-like tail, and the smaller, less prominent ones [which are a dark gray/black color] appear in places like his hair, and even form a pair of rigid cube-like wings), his arms lengthen considerably while also gaining a single spike on the elbows and his fingertips elongate into talon-like claws, his legs become double-jointed (similar to those of a grasshopper) and also seem to feature a single spike, his feet become digitigrade (meaning he walks on his toes, with his heels constantly in the air) and feature large talon-like toenails, his hair turns white and becomes more unruly and unkempt, his teeth become yellow and sharper, his tongue becomes long and blue, his ears become pointed and more elf-like, and, most notably, the Millennium Ring becomes fused to his chest with visible pronounced blood vessels surrounding it. The cubes on his left arm function as an organic Duel Disk, while his eyes can freely move about the sides of the ones protruding from his head.
Unlike his true self, who desires a world of peace without hatred, Dark Diva is obsessed with creating a world of darkness and despair, and is notably quite sadistic, as he ruthlessly struck down his own sister, and relished the idea of using a Duel with Kaiba and Yugi to make them suffer slowly, rather than disintegrating them quickly like everyone else. He also displays traits of sadomasochism, as when he took damage from the effect of Crimson Nova the Dark Cubic Lord, he simply laughed.
When Diva appeared in the same dimension Mani died in, he finds the Millennium Ring there. Without touching it, he gets engulfed in its evil. Diva is then transformed by the ring into Dark Diva, which is fused with his chest. Diva then returns, and makes everyone at Kaiba Land except for Yugi and Kaiba disappear in a wave of darkness.
Yugi identifies him as Diva, but the latter states that "Diva" is no longer in control. (In the dub, he claims that Diva has saved him from his fate, stating that "[Diva's] fear and hatred invited [him] into his soul" and that "[he] did away with Shadi Shin.") Kaiba and Yugi then agree to team up and Duel him. The Duel is a Shadow Game, where players' bodies fade away as they lose Life Points. Diva manages to defeat Kaiba, who hands Yugi the Millennium Puzzle, before disappearing. Towards the end, Yugi's body reaches its limits and he finds himself incapable of drawing his next card. Suddenly Atem appears, gaining control of Yugi's body. Atem draws and Summons "Palladium Oracle Mahad". Using "Mahad", he defeats Diva, whose body reverts to normal, before disappearing.
Dark Diva uses a "Cubic" Deck, which is largely focused on continuously Summoning and "leveling up" his "Cubic" monsters.