Yorick Unkillable Omen of Famine Ghoul

It got a kill somehow in ARAM and after that had 0/290 HP and 28 AD. It also healed me while I was alive like the normal spell does. Everytime I'd die it's AD and Healing from E's level would update. The enemy team could attack it but couldn't kill it. It would stay alive even while I was dead and follow the enemy champs around without attacking, then be by my side upon respawn.