the oath was created when the greatest of gods. Diakmanan (the god of divinity) wanted to punish the gods of the age for their poor performance. all gods where summoned together (by the hundreds) in the circle of truth, all prepared to meet their fate. that is all accept "metaldemon" (name not found in records) he thought it was not fir that they.. no he should die because some corrupted divinity says so so he forged a sword to almost completely neutralize the blast that would be used to wipe out the gods. he used the blue prints to "the oath" the great magic blade of Diakmanan to forge a weapon of similar power and similar ability's such as the drastic weakening of magic attacks (which he was counting on to stop the surge of power that would be used to destroy the old gods) but his blade was not as powerful as the original so he knew it would not completely stop the blow... he was only hoping he could withstand the rest on his own. he did. he was the only surviver of the incident later named after the weapon that caused it, "the oath".
but life was not as "metaldemon" was not happy with the new life after the oath. humans started ripping nature apart and things where going into chaos, according to him, without the dead gods. he felt guilty for saving himself and not at least a few of the other gods and since then he named titled the god of death. no one knows what duty the fallen divinity used to have...