There were only Renekton and Xerath for 1 thousand year, what happened, and how did Renekton become crazy?
ONLY a thousand years is actually quite long. Presumably Xerath worked his magic to warp Renekton's mind or he just wore down on him mentally with constant berrating of his brother; trying to point the blame to him and warp Renekton's perception of events.
Also I want to know how couldn't Renekton and Xerath find a way to escape this prison? Was it something powerful as much as Xerath's sarcophagus?
Given that the tomb was meant to protect the dead bodies of Shurima's emperors (which the sarcophagus's were also made for), it stands to reason the magic protecting the tomb was incredibly complex and powerful. I'll hazard a guess that Bard might have had a hand in things too, but that's just me spending too much time thinking about how Bard's influenced things.
And Why didn't Nasus try to find another way to prison Xerath, rather than sacrificing his brother? He could think for 100 years, or even 500 years. But without Cassiopeia and Sivir, they were left to eternity.
He probably was, or at the very least was searching for the means to just deal with Xerath in some way. He had every intention of saving his brother, and likely still does, no matter how long it takes. A millennium of solitude can make you a tad nihilistic but Nasus probably hasn't given up on Renekton just yet. The problem stands that Xerath is SO powerful that the solution to this problem is not going to be easy to find, formulate, or put in to action. Even as far as Ascended go Nasus always had the odds stacked against him in that regard.
