When matter was compressed to a tremendous extent, enough gravitational force would build up in the centre to affect atomic bonds, resulting in a fission reaction. Depending on the matter forming in the centre, the fission reaction might lead to a secondary fusion reaction or there would be a fusion reaction from the beginning.
The previous Harrala was able to cause it upon consuming the corpse of Gannala at the Dieng Canyon. She managed to create a sun that could operate for a second, which was powerful enough to kill a body of the Boar King.
Resha's powers were significantly stronger than Harrala's. He was also equipped with the Major Treasure of Cure, allowing him to wield the power of Perfect Inertial Gravity. If he had the know-how, he could manage to create a sun that could function for a few seconds at the very least.
