Decomposition of potassium permanganate under prolonged heating
Potassium manganate is produced by reacting manganese dioxide with potassium hydroxide and oxygen gas : 

$\ce{2 MnO2 + 4 KOH + O2 → 2 K2MnO4 + 2 H2O}$

So I would suspect that the reverse process will occur when further heating. (Of course when there is no water available you might get other products, maybe potassium oxide instead of potassium hydroxide)