Once upon a time, there was a captured soul that escaped a lantern. This soul wanted to have life that was so sadly taken from her so she strived to live again to see her lover once more. She attached herself to the closest thing she could find. It just so happens that what she attached herself to was a corpse of a dead deer. She was able to control the corpse but not very well so she sent a message to other nearby lost souls using magic, asking them to come and be alive again. Over the years many souls heard her message and attached themselves to this deer. She also roamed to free captured souls like herself. Hundreds of souls joined but having so much souls occupying one body they started to fuse together, staying connected forever. The deer wandered aimlessly doing nothing but collecting more lost souls until it came across the League of Legends. It was extremely skittish to anyone who came near so the League of Legends decided they shouldn't let this thing stay free potentially harming bystanders. So in an attempt to redirect it's odd behavior they used it as a champion, using it for battle.