[GAMEPLAY] Immortal Karthus (Ascension)

I recently played a game of ascension. It was all fun untill Karthus on the enemy team got the "ascended" buff. He scuicided in the middle of the map when Xerath was killed and somehow got the buff even though he was in his "afterlife form". It happened a lot of times, but nearing 3/4 Victory for the enemy team he got it again. In the same way. Suddenly he was impossible to kill. I mean like.. his hp bar didn't drop. at all. 5v1 and he won. without losing 1 single hp.