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Due to some experiments with Hextech, Jayce ends up possessed by something arcane. His eyes turn a rainbow myriad of colors, and he is no longer himself. His palms and fingers spark with magic, able to control things and do things he shouldn't be able to accomplish. The arcane being that can temporarily reside in Jayce's skin doesn't mean any harm; it will actually oftentimes try to help, but it isn't the best at helping without instilling a great amount of fear in the process.
One of the first things it tries to help fix is Viktor's illness. It recognizes Viktor as someone important to Jayce, and it also recognizes that Viktor is dying and in pain. So it goes about working sparks of magic into Viktor's body, trying to help him heal himself. Give his body the jumpstart it needs. Viktor is terrified when Jayce, who is strong and able-bodied, pins him down and starts removing his clothes. He figures out that Jayce is not fully himself quickly enough, but that does not change the terror of the situation. He is helpless against Jayce's body, and no matter how hard he squirms and writhes, he can't get away. Warm, callused hands sparking with the arcane dig into his tender muscles, and while the magical massage is ultimately helpful, it is painful and terrifying while it is happening. Viktor falls asleep before possessed Jayce lets him go, and so they both wake up the next morning with a mostly naked Viktor still pinned by Jayce. Jayce, of course, apologizes profusely, but Viktor mostly just wants to move on and figure out what happened to Jayce. Possessed Jayce hadn't done him any harm, not really, and while he hadn't appreciated the scare, he was more interested in exploring the new phenomenon of the arcane. The arcane creature now bound to Piltover's golden boy tries a few more times to help Viktor through the occasional massage, even taking him down into the forge to help keep him warm, but Viktor works his body too hard for them to make any real progress. He's dying slower than before, but he's still dying. That's when they decide to use the forge and create metal pieces infused with arcane to attach to Viktor; that way, he can have a constant stream of magic helping to stitch his body together. It works in the end, but Viktor has no way of knowing what possessed Jayce is doing. He knows nothing other than that the Arcane creature is holding him down and attaching metal to his flesh with magic.