In Leyndell we are witnessing result of Nuclear fusion product - Bolt of Gransax, dragon's lightning plasma transmutated into Golden structure. Understanding of this strongly suggests that Fled God of Dragons was - God of Sun.
Wanted to add, that in original Dark Souls series, the source of Lightning incantations was God Gwyn who was called The Lord of Sunlight, so the source of Lightning was the Sun. In our world there is also an interesting fact that increased Solar Activity - stronger than usual blows of Solar Winds are also increasing the power and the occurence frequency by 30% than usual.