Ok I need some help, my players were able to get their hands on a Holy Avenger (axe) with an unbelievable roll I gave it to them. now in about 4 levels they will be going to the 9 hells, and since this weapon will tear the hells to shreds, I need help to alter the Holy avenger when they get there. so when they enter the 9 hells their magic changes and they are watched by Asmodeus, thus making it impossible to teleport because Asmodeus will change the end location, or the wish spell he will be able to alter, so spells like that Asmodeus can have a hand in, so that might help with an idea on changing the Holy avenger.. Now I do not want to take it away because the players are really happy they got it and I don't want to be that DM that just takes things away. so I need help on how I can level the playing field when they get to hell.. Oh they will be level 13 when they go there...

Are you sure you want to change the weapon itself? You could change encounters so it doesn't make your players feel like you're taking things from them. For example, you could have an invisible spell caster or some sort of totem buffing/protecting fiendish enemies that negates the holy avenger. You could turn encounters into a mini puzzle. Let the players struggle for a bit to figure out what is going on and once they destroy the source of their troubles, let the holy avenger clean up all the enemies. 

I agree that just taking away a cool weapon would suck so I think altering the weapon might create the same effect. If you are dead set on altering the weapon, you could say the enormity of the evil energy in the nine hells has dampened the effectiveness of the holy avenger and say the bonus damage against fiends is reduced. Then, as the story progresses you can have the players encounter a lost holy relic/a secret fountain of holy water/a trapped angel in the nine hells that can slowly restore the avenger to its glory.

I hope these ideas help!

Change nothing about it - that will feel like the DM adding an arbitrary restriction punishing them for having found the item.

Instead, add a story reason why they might want to unattune from it.  That should be pretty easy to come up with - the sword, when drawn, serves as a beacon, its holy power standing out against the Hells and drawing demons toward it like moths to the flame. That means infernal creatures will exclusively attack the wielder whenever the weapon is out and the longer fights go on, the more demons might show up.  Even after the fight is over, demons might now be flocking to the area, meaning the party now has to traverse ground where demons are actively trying to hunt down the source of radiant energy they recently felt.

This provides an understandable in-story reason for the party to keep the weapon stowed.  It also adds a level of decision making - “is this a fight where drawing the weapon is worthwhile, or does it create more problems than it solves?”

I agree with Caerwyn_Glyndwer: don't nerf the weapon, that's just going to make the party upset.  In addition to being a great big "aim here" sign for all fiends in the area, remember that fiends aren't the only things found in the Nine Hells.  Try varying up encounters a bit with other creatures like Salamanders or Efreeti that aren't vulnerable to the sword.

I agree with the others about not changing the item. I’d just take it as a DM lesson about not giving a legendary item to a 9th level character. And realize that when monster CRs seem off, it’s because  the +3 means this character is hitting like it’s something like 10ish levels higher than it actually is. 

+1 for focusing on the Holy Avenger as a Plot Device, rather than worrying about the mechanics.

In a infinitely large realm, individual demons are barely more than drops in an ocean. The Holy Avenger will turn an Abyssal Chicken into spicy nuggets faster than Zariel could ask, "Would you like fries with that?", but why would they ever encounter just one? They probably roam like swarms of locusts.

Solo abilities are trumped by mobs.
AOE abilities are trumped by BBEGs.
Burst mechanics are trumped by frequent smaller encounters.
Magic is trumped by antimagic (magic dead zones, counterspell, Mage Slayer , etc...)
Melee is trumped by distance.
Ranged is trumped by obstructions.
etc...

Every basic function has at least one foil, and you should be designing your encounters in such a way that you can organically introduce a few of these as needed to control the pacing of combat. Beyond that, if you want to amp up the stakes, introduce mechanics like Reducing Max HP, Exhaustion, Madness, and Environmental hazards. Let them ride in as heroes and discover that the real challenge is just finding the next place to rest before they collapse under the strain of simply existing in a wildly inhospitable plane.

Thanks all these all help and I will plan more accordingly when they get there to have different or more enemies for encounters but making the fights more about being strategic then relying on weapons... Thanks everyone

In addition to not nerfing the weapon itself, I would also give them a fight or two in which they get to use the Holy Avenger with all of it's power before having it attract the legions of hell (or whatever plot device, etc. you end up using). It will allow the players to have some fun with it before it goes from boon to bane.

I'm wondering if you take the combined forces of Arkan the Cruel described in Descent into Avernus, both his tower and the temple of Tiamat ... how that would run.  Of course the adventure limits the encounter to white abashis'but the full chromatic spectrum would likely be available to Arkham.

Edit combining the specifically counted forces of Arkhan in BG:DiA, it's a 82800 XP encounter, adjusted to 331200 with a party of 4 13th level characters it's rated as deadly, though not accounting for the Holy Avenger.  There's a lot of undead fodder amongst them, I'd say even with the Holy Avenger that fight would cost the party something.