The unicorn was the only one to notice the shadow spreading beneath the soldier at his side, but he failed to realise what its source was in time. Only a moment before the crystal erupted from it did he finally see that it was coming from Nightmare Moon herself, as the shadow grew thick enough for him to see it in its entirety alongside its many other branches, but by then it was too late.
The spear rose from the ground and struck, and the soldier screamed in pain. The unicorn felt the magic pouring out of the crystal, and saw the root-like protrusions spreading into the other's skin where he was struck. He acted almost entirely on instinct. His magic enveloped the soldier's body and yanked it upwards by force, leaving a pulsing black wound where the crystal had been. He rested the body out of the shadow, turned to shatter the spear with a blast, then looked to the stallion again.
The soldier was shaking, his eyes shrunk to dots and the muscles in his face tense. He was in a clear state of shock and seemed unable to move his body, but it was hard to tell how conscious he was of his situation. Black roots were still spreading over his skin from the wound, which only slowly leaked out a mixture of tar-like ichor and blood. Removing the source of the magic had slowed the spread, but it hadn't halted the process. The unicorn hastily placed a temporary seal over the wound to avoid any potentially excessive blood loss, and then focused on the magic itself.
It felt disgusting as his aura reached out to it. Like a parasite eating out an animal, like some unnatural abomination that had latched itself onto a living being and wouldn't let go. It was melding itself with the soldier's own magic and with his body, and it was altering both in the process. For a moment the possibility of ripping out the affected area flashed into the unicorn's mind, but it was quickly discarded. It was a huge risk and likely to kill the soldier due to the severity of the resulting wound, and moreover it wouldn't have fully succeeded in removing the actual magic itself.
Gritting his teeth and ignoring the visceral reaction it evoked in him, the stallion began to take hold of the corruption spreading through the other's body. It wasn't easy by any means. He struggled to keep up with its spread, struggled to find every part of it as it slithered and burrowed inside the pony it was infesting, struggled to hold it and struggled against it as it squirmed in his grasp and tried to push back or inject itself into him too.
By the end of it he was sweating, panting, his own stomach not having emptied itself only because he had last eaten too long before for it to happen. With a grunt he ripped the magic from the soldier's body and hurled it on the ground in a writhing pile of twisted black appendages, then unleashed a searing spell on it and burned it away, leaving only a scorch mark in its place. Then he turned to the still trembling soldier, and began to take proper care of the wound left by the spear, which had taken on to bleeding naturally at a regular pace.