I have got a few search hits about obtaining 5 stars and 6 stars runes. As far as I remember, these runes were never obtaind by me from the Runes Summoning chamber. They were won from events, such as the recently over Demon Invasion.
These are the rewards for being at the top of the ranks.
Kanan King fragment is the lowest tier prize from this event and two of it are needed to create a 5 star rune. That was how I got my two most powerful runes. The rest of the runes here, well I guess you can figure out which one is the best of them all. One needs to get really good scores in this event to win the most powerful runes of them all (and perhaps spend the most money!) I was (only) in top 5000.
There is always a next time!