According to this thread from the League subreddit, Mordekaiser has a literal game breaking bug when it comes to interaction with Ornn.
In light of this, Mordekaiser has been completely disabled in champ select until the bug is fixed. Mordekaiser is now re-enabled on the live servers and this bug has been fixed.
If Mordekaiser kills Ornn with his ultimate, the game crashes and completely disappears. The game is gone for all 10 players, with no trace of existence.
Some users in the thread have pointed out that Ornn’s model might not interact right with turning into a ghost, but we don’t know for sure. Something spooky is definitely going on.
Riot knows what’s going on, at least, so hopefully it doesn’t take long to get Morde back on the Rift. They were pretty quick to disable Mordekaiser, so hopefully not too many people had games ruined because of this bug. We’ll have to wait to see what the cause is as they haven’t said anything else about the champion.