Unkillable!!! [GAMEPLAY]

Recently i played an ARAM game and suddenly a wild cow appeared, an Alistar was untocuhable, i mean, none could deal damage to him (even turrets), he was tanking our turrets and stacking minion waves without being able to move with his will or damage anywone, after 5 minutes, he went back to his base and returned to his normal form, after that he said something like: "Wtf xD", and that was his last word he said, so i dont know if this is a bug or a hack, but it seems really impossible, especially in this season... https://www.youtube.com/watch?v=zdOo3bHdvK4&feature=youtu.be

P.S One is sure, be ware of the , sorry for the low quality video, my PC is on low specs (Special thanks to my brother who recorded this)