Sombra/speculation
With apologies to the good Lich
SpoilerWait a second. There was no statue of Sombra. He was just gone... banished if you will, by the elements of harmony(the first time). Maybe he was actually possessed/corrupted by the whispering forces of darkness?
Maybe there was a prophecy that he was going to be replaced by Cadance and he just snapped to hold onto power. So in order to hold off this prophecy, he curses his own kingdom as he goes into banishment. At the same time as his failure, the aforementioned whispering forces of darkness shift to try and get into Luna's mind.
Ooh, delicious hints of a greater evil!