The First Gods were immensely powerful beings before the awakening of Vennar. To the majority of cultures on Vennar, the First Gods are entirely unknown and unknowable, and are only to be found in the darkest of forgotten texts of madmen. It is only through centuries of utterly skilled research or the blessing(or curse) of the current gods that the First Gods are known.
Unknown even to the greatest of scholars, the First Gods were the incarnations of Vennar's first thoughts as a newborn planet. Love, Hate, Happiness, Sadness, Pain, Pleasure, all of the most primal and overarching of emotions, thoughts, and ideas felt by a child of extraplanar origins before it became self aware. It is from the First Gods that the celestials and infernals spawned, the lesser mix of the greater.
As the First Gods came into being and spawned their legions of celestials or infernals, the primal beings warred and fought with each other, squabbling over the conflict inherent in their very essence. These squabbles spread over the face of Vennar, causing more havoc than the already tumultuous ordering of her body. Their spawn turned on them, as they turned on their spawn, and over the vast, unknowable millenia, the First Gods slaughtered each other and were slaughtered by their very creations.
The First Gods were very primal and ultimately simple beings. Their entire essence consisted only of the most overarching and, ultimately, basic of thoughts and emotions. The First Gods warred with each other because it was in their nature. Love and Hatred did not exist in calm tandem with each other, and so they warred. Pain and Pleasure mixed and warred together, so utterly different and yet sharing traits far similar than the others.