Hey, All! (Long-ish post ahead)

I am wanting to create a campaign that is Deity based...meaning there will be piety points, the potential for a deity to be the arch-villain (that is my goal but it will also depend on the players at my table, etc.), etc.! I have the Mythic Odysseys of Theros sourcebook that I am using as my basis. (I am still deliberating if I want to use the pantheon provided in MOT or to have a "war" happening between the Greek, Roman, Egyptian and Norse pantheons.) My question: What's the best approach to have a deity as the arch-villain?

I have one player who is absolutely convinced that if any champion kills a singular god, the rest of the pantheon would do everything in their power to destroy the mortal because "no mortal should wield the powers of gods." However, my end goal (not saying anything to this player and the others) is that the champion who delivers the finishing blow will be offered the duties and mantle of the now deceased god and the others become demigods and legends. (For example, if Athreos, Theros' god of passage and the River Guide, has become unreliable in his duties to keep balance between life and death so Heliod and Thassa send champions. Thassa champion succeeds in killing Athreos so the gods give the character –by DM offering player– the opportunity to become Athreos and become the new River Guide. If player says no, the offer is then given to the party. Once Athreos' position is filled, the campaign ends.)

There hasn't been much I could find in the DMG on the subject, and MOT offers deities as villains as an option. Am I missing something and are there other resources and rules that I can refer to?

EDIT: I should add... This player (that inspired this question) is an atheist and I am a Christian/Pastor. He's played a paladin before in our group and has been playing D&D for 13+ years so he is familiar with the mechanics of D&D, more so than me. However, as a Christian, my views on the relationship between humans and God do color my approach to the relationship between deities and mortals. My favorite class to play is "Cleric" and my pantheon was heavily involved in my last campaign as a DM (they kept popping in and out like Gandalf in the Hobbit...it took my players a couple of months to figure out that the guy they tried to murder on their first encounter was a deity who was there to help). Therefore, I am excited to use MOT in the campaign I am currently building. I am also a DM that heavily encourages RPing.

I wouldn't give a singular player the power of a god. In fact, I'd say that the power of a god should not be transferred upon death, it should only be safely transferable via the god willingly giving it up, as Hestia gave up her throne to Dionysus. If a god is slain, it results in its powers being scattered, only partially transferred, like what occured when Cronus slew Uranus, scattering his blood all over the world and giving birth to all sorts of monsters. Instead, the best way to go about things is to imprison the god and force it to give up its power. For example, the titans were imprisoned in Tartarus after their defeat, and while this isn't really canon, perhaps they gave up their power willingly to their children, the gods of Olympus. I'm using greek mythology here because MOoT is blatantly stolen from it, so it's basically the same thing.

I would say the best approach to making a god a villain is perhaps they are abusing their powers, or attempting to rise above their station. Maybe your charon equivalent isn't happy with just being the ferryman, and wants to rise up and take the underworld for himself. Or perhaps your Hades equivalent is attempting to take the sea and the sky as his own, for he was never quite happy with the lot that he drew. Either way, the gods would most likely send their champions to go put the god back in their place. These champions would be the PCs.

That's not a long post ;)

Having a God as the core villain in a Campaign is tricky and ambitious, but not really all that different from any other villain at the core.

My advice would be threefold:

• Makes lots of general notes as to what you envision this Campaign involving, but don't cast any of it in stone yet. You probably won't need to set all of it in motion until around level 18 or so ( and sadly not many Campaign make it to the end ).
• As part of the overall campaign notes, figure out what is the core conflict about. The villain/diety wants something, and the Party is set up to oppose them. The villain will have a goal and agenda.
• Start small. Don't start with the God. Start about three or four levels down God > Religion > Temples > A particular Cleric or Paladin. That will be your initial Villain. Figure out what they want, and why the Party would get involved in stopping them. 

Now your initial villain's goal will likely be shaped by the overall villain/deity's goal, but none of that will be visible to the Party at all.

So far as the Party knows, they're just facing a mortal - if powerful - adversary on a local level. As the conflict continues, expands, and unfolds, the Party will get involved with higher and higher levels of the organization, like peeling an onion in reverse, until the final showdown will be the Party storming the halls of the Gods themselves :)

Keep your eye on the overall Campaign shape, but build and play only the part that the Party is interacting with - and then the next, and the next.

You will also find that your high-level general Campaign notes will evolve over time, and especially with the Adventures of the Party. 

The Campaign you end up building and playing may be very different than the one you initially make high level design notes about :)

 Edit: An addendum - the end you're picturing should probably be the end of the Campaign. Godlike Characters are probably not playable in this system.

Maybe instead of kill, just defeat the god. Gods can (in rare cases) admit defeat.

I suggest, for reading, since it is both instructive and a damn good read, the George Perez Wonder Woman series from the 1980s. In particular, the first 6 issues, which represent the first story arc, are some of the best comic-books ever written (and especially drawn, since it was Perez at his height), and has Wonder Woman going up against Ares. (This is where the WW movie got its vague plot, although the movie is nowhere near as well written as the comic.) In the end, Diana fights Ares, and in victory she forces Ares to admit defeat. But she does not kill him. I won't say anymore for spoilers -- read it for yourself. It is awesome. And very instructive if you want to use gods as antagonists in a story.

My point here is -- defeating the god need not mean killing him. It might mean imprisoning, de-powering, or just getting him to stop whatever it is he (or she) is doing.

If you want a God to be easier for the party to interact with you have to bring out the big guns...

The Time of Troubles Two Electric Boogaloo!

Just have the Gods do something that pisses off Ao so much they're forced back down to walk alongside their people once again. Or you could forgo the Faerûn history and make this the first time it's happened and change the god who has overseen this drastic change. Maybe Ra, Odin, and Zeus were all fed up and worked together to punish the other deities. It's a lot of fun building the game world around this concept of the Gods let loose among mere mortals!

Quote from BioWizard >>

My point here is -- defeating the god need not mean killing him. It might mean imprisoning, de-powering, or just getting him to stop whatever it is he (or she) is doing.

 Aye. Take a page out of Otus and Ephialtes' book and yeet the lad into an urn.

That's an excellent idea! That, or something analogous: some time, conjunction, item, or location, which puts the God and the Players on a more even playing field.

This Arcane conjunction only occurs once every 3,141 years, but on that one night, the Gods are vulnerable to The Crossed Dagger of Ajanti.

I say go for it if your characters are a reasonable level,ive never done a god in any of my campaigns though, but I am doing a campaign where the enemy a lizard folk king though.

Also, if you end the campaign on that note, with the god being killed, then the consequences won't matter that much. Campaign over, time to start a new one, and the killing of the god at the very end is something people will talk about for years.

They way I do my gods is they have almost unlimited power but limited times they can use this power. How ever they can take a ritual where they lose most of there power in exchange for appearing on the world they can reach god hood again but they need a very powerful object that relate to there domains. If they manage to complete that then they get full godhood again with the power to use as much as they want. If a mortal manages to kill a god they become a Demi god but can’t ascend with out the help of another god.

What does the god want and why can't she get it?

First, I’d say ignore what your player says about stealing a God’s power. There is nothing RAW about what powers a god has and how one might go about killing/supplanting one, which means how it works (or if it is even possible) is entirely up to you. 

For something to work on a campaign length arc, you need something to nerf the god or just generally keep them handcuffed. Otherwise, the god can kill the mortal at will, and quite easily (anything from giving the character a heart attack to dropping the tarrasque in front of him). So you’d need there to be some taboo or rule the gods must follow to prevent them from interacting much with the mortal realm. Alternatively, you could make a character the favorite of a different god, so the villain god can’t act directly without starting a god war. 

But I’d stay away from the character as favorite option, since in that case, the dice will one day decide that character will die, and the premise of your whole campaign is shot. 

And I’ll agree with the others who said that killing the god marks the end of the campaign. But the fallout from it happening sure sets up some interesting options for the next campaign.