Osirion did not actually want this at all!

Some of them, at least, were expecting that it would happen eventually.

What kinds of spells affect transparent tortured soldiers. Can you turn them? Heal them? Frighten them? Plane shift them? Paralyze them with waves of ecstasy (look, some people didn't prepare their spells this morning with an invasion from Hell in mind).

Turn them: No, they're not that kind of undead. Heal them: Oooh, yes, thank you kindly, they appreciate the boost! Frighten them: No. Plane shift them: Sure! And then five seconds later the very same soldiers are back. Mephistopheles laughs at that last one, a great loud booming laugh that echoes to the nearest city. Well, that's delightful, but no, his soldiers are just not in that kind of mood, but he's sure they're flattered.

Would they like to try anything else? This Archdevil is having so much fun.

Teleport, Polymorph, obviously lots of smiting evil and so on, Imprisonment, Mage's Disjunction, Mass Suffocation, someone can Wish that they were gone entirely and if that doesn't work someone can Wish they were in the middle of the ocean and if that doesn't work someone can WIsh they were invading Heaven even though perhaps it could be argued that's a fucked-up thing to wish or something...

Teleport: Yes, and then no, rather like the plane shift. Polymorph: Yes, and then the fire rain lands on the polymorphed creatures, and then no. Smiting evil: Yes, and then no, when the soldiers 'die' and then come back. Imprisonment: No, they decline to stay imprisoned. Mage's Disjunction doesn't even tickle. Mass Suffocation doesn't appear to do anything at all. First Wish doesn't work, second Wish briefly works and then doesn't. Third wish does nothing at all, Heaven has defenses against that sort of thing, and my that's entertainingly counterproductive, what would they have done if he'd conquered Heaven, hm? They'd be much, much worse off. Not that he'll tell them.

Someone would probably scold that mage if they didn't need all the help they could get. 

(Wishing they were invading some country on this plane that isn't Osirion is considered but it is decided this is really a pretty evil thing to do and this is the sort of moment where one should keep an eye on where they want to end up.)

So the fire rain seems like part of the problem here? Is it responsive to, like, freezing spells or weather-altering spells or being teleported off or shifted into other planes or any of that?

Freezing spells: Some, but not enough to really matter. Weather-altering spells: No, it's a bit too hot to be put out that easily, though there certainly is some ominous smoke going on now. Being teleported works for the specific fiery droplets, but not enough to matter. Same for shifting to other planes. The source of the fire rain seems disinclined to go away, no matter how much fire rain they happen to directly put out or avoid.

It's potentially an opening, though, which is something.

Honestly they ...need Abadar to show up. But maybe he's busy and they should try to mitigate the damage in the meantime.

If they simultaneously put out a lot of fire rain and smite a lot of evil those ones stay dead until the rain resumes?

Nope! They come back right on time, hearty and tortured as ever.

If they put out a lot of fire rain and do a lot of polymorphs?

That has some effect, actually! But they have to keep putting out the fire rain, and keep all of the polymorphed soldiers in one spot, and—

—then Mephistopheles himself shows up and sets everything very much on fire, and the soldiers are back and oh wow that's a lot of death right there oh right it's not just the army they're dealing with, is it.

Well, they know (one thing) they need to do. The problem is just that they...cannot do it. They don't have that many high-level casters and other countries' adventurers are mostly sensibly staying out of this one and people who aren't sure of their alignment should maybe...board boats? 

That does seem wise, yes.

Mephistopheles lovingly rains some fire on the boats, just because. And really, if you don't take the time to enjoy your job, what's the point of being evil?

Nine hours into the invasion, and the pharoah of Osirion gets a sending from an unfamiliar person: Epic help incoming. Push enemies to tiefling. Stall and prepare until we make our move. Wall leftovers underground with stone once we’re done. Conserve forces.

Well that's better than anything else he's heard today. It is of course completely possible that it's from Mephistopheles, but it is possible that it is not. Which would be -

- are there any tieflings to push his enemies towards, if he sees a way to do that?

No, just a lot of invincible soldiers made from the tortured souls of the damned, no tiefling obviously making himself appar—

He appears out of the sky in the form of a giant, crashes onto the most concentrated part of the battlefield, pounds his fists into the ground to deepen the crater, and then his polymorph drops and it becomes very clear that the form of a giant was actually much less scary than his actual form.

Yes, this tiefling is worthy of an article before his pedigree. He is definitely epic level. The invincible soldiers are still quite invincible, but he's fast and deadly enough that it doesn't matter, they get dusted as soon as they're done reforming, and then he's on to the next, and the next, and the next, and back around again, and all of these annoyingly invincible soldiers can just stay right here with him in this crater, invincible or no.

This is still as likely as not to be a trap but - they don't have anything else to try - 

They can attempt, with a lot of stone walls mostly, to herd enemies that direction.

Mephistopheles lands in front of the whirling tiefling, wings flared, and begins trying to personally kill him.

It does not take.

Snarling like an animal, eyes glowing red with demonic rage, the tiefling fights like a man possessed, with no care for safety or tactics, just kill the thing with as much prejudice as possible.

Then, just before it seems like he might fall despite all his impossible strength and maddened rage, the tiefling breaks something in his hand, murmurs a word, and disappears.

From the sky, arcane words are spoken, and a gesture is made.

Below the wizard, all of the army in a forty-foot hemisphere gets turned to frogs.

Mephistopheles resists the epic spell, somehow, glaring up at the invisible enemy in the sky.

"You again! Can't you just take a hint and stay on your plane?"

"No," says a patch of air, "not really."

And then she conjures a great celestial triceratops, which starts trying to skewer him.

"Done!" yells a chorus of illusionary voices.

Wall everybody underground with stone, they said. 

They can do that. 

Good.

Several minutes later, the tiefling, looking rather a bit more bloody and haggard, turns visible just as he is deposited gently on the ground by the pharaoh's forces.

"Never," he mutters to the air, "would I have expected to miss the kobold."

The air snorts, despite herself, then gets back to business. "Cleric!"