PvP or PvE?

Rank 0 - Tutorial
The tutorial will consist of gaining basic familiarity with controls. You might attack something, but it won't attack back.

Rank 1 - Apprentice
The apprenticeship period is spent learning the basics of your profession. You may be attacked by beasts, but you are safe from other players, even the gods. You aren't worth their attention.

Ranks 2 & 3 - Freeman, Master
You become a target for anyone. You are capable of attacking anything rank 2 through 5, and vice versa. You also are vulnerable to the eccentricities of the gods. This is the most vulnerable period of your life.

Rank 4 - Hero
You are mighty enough to battle demi-gods, and likely your epic journey to immortality will involve the defeat of one -- well as much as one can be defeated.

Rank 5 - Immortality
You are now incapable of being killed by anyone below rank 7, though you may still be wounded by them. Your progression of skills ends here; at this point your focus switches from gaining experience to gaining followers. The beasts of the land are no longer your concern (you can't attack them), except for perhaps those really nasty things that are unleashed upon the world by the fickle actions of gods.

Rank 6 - Demi-God
The ascendence to demi-god involves the transition from the affairs of men to the affairs of gods. You will spend most of your time the land of gods, venturing forth to the mortal world only for epic situations. You can't be killed, but battle with the gods will tax your powers and weaken you.

Ranks 7 & 8 - Lesser God, Greater God
You power is derived from alliances and followers. You cannot be killed (unless you lose all your followers and return to demi-god status), but again battle with other gods will tax your powers.

Rank 8 - Supreme Deity
You are at the pinnicle of the game. Unfortunately this means everyone else is gunning for you. Your power can't be drained as it is derived directly from the nine greater gods (with or without their consent), but you can still be weakened in battles with the other gods. If you are sufficiently weakened and the majority of the gods vote in a new supreme being, you are utterly destroyed.


Rationale
Lowest ranks are for people learning the game and getting started. They are removed from basic griefing for now.
Middle ranks are the bulk of the game. The restriction is nation vs. nation, but since nations often align themselves with gods, it will often be more of a holy war.
The upper ranks are the advanced part of the game. Killing critters is boring now, so you're in it for combat with others. However you spent a lot of time getting to this point, so it shouldn't be easy to make you disappear in a puff of smoke.

Why kill the supreme being?
This is the pinnicle of the game. Reaching here, even if it doesn't last long, constitutes "winning" the game. If you are deposed, its time to start over rather than flounder around with a character you've spent way too much time on.