actualy there is a history of 2 evils that teamed up against the prime evils. i know all about is now hehe
ok this is how it happend.........
there is a war known as the 'great conflict' and it is a war that has been going on for billions of years. this war is to see wether it is heaven or hell that will control all creation. now when this war falls upon mortal soil it is called the 'sin war'.
meanwhile somtime in time<i will refer to around the beging of diablo 1> around 300 years ago the three prime evils were overthrown by 4 (known) lesser evils, the reason for this was to gain more control in the sin war and take over hell for themselves. the three were banished to earth in a dark exile. the three reaked havoc upon the western kingdoms and finaly a group of powerful magi was formed to stop the three once and for all. this is a group know as the 'Horadrim'. the horadrim chased down mephisto and bound him in the temple city travincal. Baal was a much more powerful demon, they could not bound him in the soulstones that tyreal the archangel had given them to contain the demons. so the brave and powerful horadric mage 'tal rasha' humbly volonteered to contain the demon within himself, to ressel with baal for all eternity. the third demon 'diablo' was chased all the way to the eastern kingdoms. they finaly cornered him and imprisoned him in a deep labyrinth that was once a great cathedral built in the sin war.
mehpisto was the first captured and in turn the first to corrupt the soulstone. diablo was the second, he corrupted the land around him and the king leoric, and his preist lazarus. king leoric was to strong for diablo to capture so he tried to posses his son prince Albrecht. many warriors tried to rid the land of evil but the warrior that finaly reached his destiny was so twisted by diablo that he thought the only way to stop diablo was to contain the demon within himself, therefor completing diablo masterfuly done plan.
One of the lesser evil's that overthrew the 3 primes evils was called to do diablo's bidding once again. this lesser is known as 'andariel' maiden of anguish. the other evil is know as 'duriel' who was called forth to guard the entrance to baals tomb as punishment. durel also is know as 'the prince of pain'.
now the 2 lesser evils that actualy conspired against the three and asked andarial and duriel to join them are known as 'Belial' and 'Azmodan'. there is not many information given about them but the fact that they are refered to as 'lords' themselves. therefor logicaly assuming that they are capable of rivaling with the three brothers for power stuggles.
remember even though it sounds like hell has turned on themselves, this is just an injustice of a power stuggle to see who can control hell. the evils belial and azmodan are very evil and just becuase they banished there own brethren to the mortal plane does not mean they want to do good for heaven....far from it. the move was made to actualy get a lead on the sin war and destroy the mortals that are helping the light in this realm, and to gain more power in the ranks of hell.