Vegum, Lily and Darla, shoo! The rest of this is not for you.

Okay hive mind, I could do with some suggestions on how to proceed with my campaign. Some background is necessary. I'll try to keep it concise.

Three sessions ago, the PCs successfully aided a resistance group in Usaro to overthrow The Faithful, a faction of charau-ka who were waiting for the former Gorilla King Ruthazek to reincarnate. As per 2e lore, the Altar of Angazhan was stolen from Usaro some time ago, and its loss weakened them quite a bit. Another faction of charau-ka, The Unheralded, have left the city in search of the altar. They want to see a new Gorilla King take over.

With the Faithful scattered and the Unheralded gone, the PCs decided to raid the Vaults of the Gorilla King in search of sweet loot. They got a ton of gold, art objects, magic items. And they found the battering ram Worldbreaker which an ancient Gorilla King stole from the 5th Taldan Army of Exploration.

The lore splatbooks are vague about Worldbreaker's abilities. So I had to homebrew it. In short, it's a battering ram made of ironwood reinforced with adamantine which can be wielded by four medium creatures of Strength 16 or greater. It deals a crap-ton of damage to any object you hit with it. And just to make it extra badass, I decided it's a minor artifact that casts a targeted Mage's Disjunction on any object you strike with it. You know, for knocking down pesky walls of force and such.

There's just one thing I had not really thought through: Mage's Disjunction can be used to destroy artifacts. Like, say, the Altar of Angazhan. Ordinarily super-high-level spellcasters are probably going to be pretty loathe to risk losing their spellcasting ability by blowing up an artifact with Mage's Disjunction. But in this case, the casters would logically be the people wielding Worldbreaker, who might well be a bunch of fighters who have no ability to cast spells and won't miss what they never had.

When that came up in play following a good Arcana check to know how Mage's Disjunction worked, the players immediately zeroed in on the possibility of putting a final end to the line of Gorilla Kings. It was already established that Gira -- the head of the resistance in Usaro -- helped arrange the combined Matanji/Kallijae raid that stole the Altar in the first place. She hates the Gorilla Kings, has an in with the people who currently have the Altar, and desperately wants to destroy that Altar. So when the players brought up that possibility, I felt I had to honor those in-character motivations.

I was hoping the party might agree to just give Worldbreaker to Gira as her share of the loot for the Vault expedition. But the PCs -- Vegum in particular -- just flat-out did not trust her with such a powerful thing. Over the course of a real-world hour, he argued the other two to his position.

So now the party has committed to taking Worldbreaker to the Kallijae and using it to destroy the Altar of Angazhan.

Did I mention they're level 8? Fighter, Witch and UnRogue 5/PF Chronicler 3.

...

I am struggling to figure out how to set up a plausible scenario for this. Something as world-changing as destroying the Altar of Angazhan should be epic. The CR of some kind of encounter to make this happen should be way over their level. Like, Angazhan is not likely to sit back and let a pack of pesky mortals blow up his Altar unimpeded. He wouldn't show up personally, but I can easily see him sending a bunch of baregara or similar.

I can absolutely give them allies and such. Per canon lore the Altar is currently in the possession of Dayo Heartshaker, a level 11 warrior (in PF2e terms), and she would logically have a bunch of similarly leveled allies who helped steal it in the first place. She and her allies actively want to destroy the altar, have it in their possession, and are eager to welcome allies who can help them blow it up. Honestly, this is a golden opportunity to be rid of it. But I worry about upstaging the PCs by filling the scene with NPCs who are higher level than they are.

In short, I want to give my PCs a challenge they can plausibly succeed at (even if it's really damn hard) without throwing in a bunch of cheap deus ex machina stuff.

Any suggestions?

TL;DR: My level 8 PCs have chosen to pick a fight that should logically be CR 15 or higher. I have wiggle room. What are some ways I could plausibly make this feasible?