I know that each god will send his Avatar to kill you if he spawns, but does each god have his own Avatar or do all of the gods use the same Avatar unit?
Also, how powerful is the avatar? Can a single Avatar destroy your whole empire? What can they do?
Also, if you kill the Avatar and doing so is not a victory condition, then will that god's avatar be gone for good? Will other gods still be able to attack with their Avatars? And can more then one avatar be on the map at the same time in case more then 1 faction has angered a god enough?
The Avatar is very powerful, but is unlikely to ruin an entire empire. It's basically a hugely jacked up temple unit.
The Avatar of Lunord, for instance is an Adept of Lunord, but with +75 to all resistance, 1000HP, something like double damage, flying, +50 regeneration, and attacks in a 3 hex arc. They also level up just like any other unit.
Avatars show up as a quest creature with a victory reward of "you win". I don't believe that they can show with the victory condition turned off. Theoretically, you could have multiple Avatars on the map, but the AI so rarely builds temples tht it is highly unlikely.