(Well, I wasn't going to write this now, but I already had my own interpretation, so here goes. Please notice that this is a very rough draft.)
"Kaiba, why did you do that?", called out Yugi to the six-foot tall teen. To which the Ceo replied: "For him. The Pharaoh." With his gaze no longer focused on the game against the maleficent ring possessed Diva, he continued, handing the puzzle to the smaller duelist: "Take this...". Sensing his energy falter, the brunette collapsed, but not before a final request while he was fading away to another dimension: "Call him...Bring him back, Yugi..."
The younger teen gasped in shock: "But Kaiba...The Pharaoh is gone!.."
The mad possessed Diva was taunting the youngster, mentioning it's plans, while Yugi proceeded to put on the Millennium Puzzle, a long lost habit he wasn't counting on doing again. The pendant shone, as if it was sending a plea, a distress signal, or a warning to someone.
At the Afterlife palace, a crimson eyed teen pharaoh was attending the kingdom matters of the day with his court priests, when suddenly he sensed something squeezing his chest. Paying closer attention, he gasped. "The Puzzle has been found and reassembled? This is not good...I wonder what is going on..." Deciding that it was better to not continue with the court session, he dismissed it, yet keeping Mahado nearby, for only a magician would have an explanation to what was happening. Eventually both found out about was was going on in Domino, Japan. Atem couldn't help but worry, but was pleased in seeing the teamwork of his former rival and his eternal love. He would not interfere. If there were anyone who would defeat that possessed demon, those two would be up to the task.
But then the Ceo collapsed and vanished. And his Aibou was fighting alone, becoming exhausted by every blow and fall, yet struggling the best he could manage.
"Mahado, be prepared if we need to interfere. I sense that Aibou is in danger, yet he has not exactly asked for help...", commanded the pharaoh.
While everyone around was fading away, consumed by the darkness, Yugi tried to keep his composure and to draw another card. However, exhaustion took over, and the small duelist felt dizzy. Apologizing to everyone, he started to collapse, but not before musing to himself: "I guess I'll be seeing you in a while, Mon Hitori no Boku..."
That was all the crimson eyed teen needed to hear. "Mahado, it's time, let's go! Aibou needs our help!", said now a frantic Atem. In a beam of light over his loved one, he arrived just in time to avoid the smaller duelist hitting the ground, entering once more the other's body, and catching him in his arms at the soul room's corridor. "Okay, Aibou, I'm here. We've got this. I brought Mahado with me. Let's do this together, ok? Let's put that demon down. I know you're not strong enough now, but I will borrow you my strength."