yeah you heard me right, GOD. they have killed a third of one of my countries, and hates provolone. an. absolute. nightmare.

Once a character goes beyond the sensible boundaries of a player character, you should turn them into an NPC and get them to roll up another character. If this happened at level 20 (as I sincerely hope it did) then briefly give all the players closure on their characters and start a new campaign. Same deal if the PC becomes a Dragon, or some other equally powerful creature that makes them the centre of the game. It's not fair on the other characters if they have an all-powerful god-bard and the rest are just normal PCs.

Start a new campaign set in said ravaged country, with a premise of "you all seek revenge on the same god", and then get them to kill the god-bard.

If this is a brag about how wild your game gets, then congrats because those types of games are always zany and fun. 

If this is an actual ask for help in this situation, then as the DM you should just say 'no'. If the bard says "I want to roll to see if I can kill God", you respond by saying "that's not how this game works. You CAN roll to attack that giant rat if you want though". Say no and give them opportunities to do things that they should be doing, instead of 'yes-and'-ing yourself into a corner. 

you see, the player is level 9. and they are progressing faster than i can rebalance their path, which is homebrew i am yet to post here

I feel like the only way a level 9 character can transcend a god is a failure to say "no, that doesn't work". 

What did they do to become so powerful? How did they kill a thrid of a country? How do you run sessions - do the other players just talk amongst themselves?

If a level 9 character has grown out of control, that is very likely the result of DM issues - giving out too many items, not knowing how to balance encounters, misunderstanding something about their class, not imposing consequences to actions, etc.

Take, for example, they’re killing a third of a country. You could have had a large group of soldiers of some high CR monster (styled as a knight or some other hero, or perhaps a guardian protector like a dragon or such) show up and kill them.  Fortunately, this is a solution you could still employ, though you’ll need to deal with some of the fallout of their actions. Given the scope of what you permitted them to do, that likely is going to force major changes to your story. 

Another option is to abandon your present campaign plans and move them somewhere completely different, allowing you to mulligan and start over, regaining some control over the situation.  For example, perhaps a demon Lord saw what they did, was impressed, and now wants them to do some task in the Nine Hells.  This can be hard - you have to make up a whole new schtick - but it also gives you the most freedom in creating a hard break and distancing yourself from existing issues.

basically its an action that allows their stats to triple for a turn, but it became detrimental when they used it while not in combat.

also it was the capital city of a small country

that shouldn't make them a god by any means. Capable in almost everything, yes, but they won't have divine power.

how often can they use this action? If it's whenever they like, then I would retcon it. Nothing wrong with saying "as you use the macguffin of oomph, you feel it crack." and then explaining that it has 3 charges now, and regains one per day.

It sounds like you could probably benefit from some reviews of your homebrew material. Tripling your stats at level 9 is way out of the scope of normal class features, and adding stats in general is not really a common path used for temporary buffs. When you try new paths like this you often end up with unintended consequences.

Also in a more general sense, a feature that grants an amazing buff but also some sort of penalty to "balance" it is always going to be a challenge to players to bypass or ignore the penalty. It makes the feature much more prone to abuse.

well the ability was given through a rune which bound to their soul, they do only get 2 uses per session, because our sessions are one hour at a time, im thinking of scaling up the BBEG to be more reasonable to their skill level

I failed to mention that i'm not using a module. I made the world myself

Also, the continent that was damaged. its going to revert back to a state before it was destroyed once the party obtains The Scroll of Comprehension which might fix my "bard problem" if you've read the item's attunement requirements.

Im thinking on making it so after the 2 uses have been expended, the player drops to 1 hp

"uses per session" is a bad way to go as a session could span several in-game days, or about 3 in-game minutes.

Instead consider "once per long rest", make it last for 1 round, and give him a round of doing nothing (like from Haste) afterwards.

consider the Magnificent Guild of Homebrew to post your homebrews and get them reviewed by other DMs so you don't break your game again!

I'll consider it, but Im contemplating if i should just kill them off or let them kill themselves. 

Killing them off: Nearly impossible for natural causes, they have Ailvilaroch which can predict the future

Letting them kill themselves: possible, but very dangerous. could cause more imbalance , because of the scroll of comprehension (mentioned previously)

killing PCs is an unfun solution because it's a forgone thing - you declared "they will die" and as you are the DM, it's going to happen.

better to avoid all of that back & forth and emotion and just say "ok, this is too powerful, I nerfed it, deal with it"

yeah im probably gonna nerf it 

most of my homebrew posted here is me re-balancing things that by some random chance got in my players hands

This is a great example of why if you don't want God killed, don't give it stats 

Quit home brewing magic objects,and abilities.

Its fun to go crazy for a campaign once in a while but make sure everyone knows its just for laughs and will not last long.
Sort of like a Christmas one off.

Quote from TheGnome5786 >>

Quit home brewing magic objects,and abilities.

To OP, this is ridiculous advice and you should ignore it.  Homebrewing has been an important part of D&D from the game’s inception, and you shouldn’t let one (admittedly big) mess up on your part ruin that. For something that’s actually helpful:

When you are homebrewing, look at what was already published and use that as a baseline for power.  There is nothing like what you gave your player - and for good reason which you have now learned.

Homebrewing items and abilities is fine, but you need to be very careful or they can break the game before you give them to your players.  By looking at what was already published and not making something more powerful than that, you’ve a good idea of what power level will work.

And, if you’re not feeling confident in comparisons, do a pick and choose method - rather than invent something new, take abilities of existing items, feats, etc. and mash them together to make something new.  You have to be careful with that as well, especially since you could make inadvertent combos,  but it still is a tad easier than trying to evaluate something you made from scratch. 

Yeah, I've talked with my players about taking some time to re-balance the campaign, and they are understanding and expressing aspects they don't agree on.

also, taking the time to compare and change stats will majorly benefit the story, Thanks for the advice