The legends in Remnant say that it is a rare occurrence that a human would run dry of aura and turn into a creature of Grimm over a certain lapse of time, Team RWBY didn't believe this. They thought that Grimm where just monsters, not actual people, to them, the closest to human Grimms were the Faunus.
It was morning when Glynda had found a few White Fang members hiding in the Emerald Forest and Blake was the first to head out, Yang then followed, leaving Ruby and Weiss in Beacon, Blake was her partner.
Only five minutes behind the Faunus, Yang tried to catch up until she heard a scream, "Blake!" Yang called out, sprinting to where the scream had come from, thoughts of those White Fang creeps killing her filled her mind with anger and sadness. There she was, near death, there was no other thing to do, "Y-Yang?" Blake murmured, "Ssh, I'm trying to save your life, stay quiet," Yang said softly, a hand was placed by Blake's heart, she felt a faint heartbeat.
"Wait, Yang, don't do anything you'll regret..." Blake saw Yang glow; she was using her aura to revive Blake, "Yang! Stop! Ozpin said..." Yang cut in, "Shut up Blake. I don't care about the consequences right now."
Blake felt slight energy return to her while Yang, on the other hand, looked like she was almost out of aura, "Yang! Stop now, I'm fine, look at me," Blake begged the blonde, Yang was in pain, her normal lilac eyes had turned red, Blake thought it was Yang just getting worked up like usual, but Yang was screaming.
"Blake! RUN!" Yang cried out, Ozpin was right, this meant that Yang was turning, "There's still time, Yang, I'll get you back to Beacon and Ozpin will fix this," Blake reassured. Yang smiled, "It's been awesome hanging out with you Blake, just wanted to say that before I become a monster."
"Y-Yang. Listen here, right now. I'm going to get you back to Beacon, so you can see Ruby once last time; your sister would like to see you go, not in that way, but..." Blake began, "But what? Let her see me turn? No way Blake," Yang said, struggling to retain her humanity, she inched slowly to Blake, who had the strength to stand, "Just one thing before I go," Yang smiled as she pressed her lips against Blake's, the Faunus' eyes closed, tears ran down Blake's face. When her eyes opened, Yang was gone.
Ruby and Weiss grew worried, neither Blake nor Yang had come back, "What if the White Fang…" Ruby began, "Don't talk like that, both Blake and Yang could handle Faunus ruffians, no difficulty," Weiss interrupted, then, they heard familiar footsteps, "See Ruby, I told…" Weiss paused as Only Blake had returned, badly injured, but still standing, "Y-Yang?" Blake looked away from Ruby, "Blake! Where is Yang?!" She demanded as she was nearly in tears; Blake looked up, "Gone," she said quietly, "Speak up, Blake," Weiss growled, "She's GONE! Ozpin was right! Humans turn into Grimm! Yang is a Grimm!" Blake cried before collapsing.