The 4 members of Tau 5 (Irantu, Nanku, Munru and Onru) step though a deminsional rift, appearing on the other side in an alleyway, they are halted by a squad of blackmesh troopers, what happens?

Tau 5 has their standard loadout of foundation weapons and armor.

The Blackmesh have their own standard weapons and nanosuit armor.

7610041
This is easy to answer!

Irantu, Nanku, Munru and Onru come from the universe of the SCP Foundation. Magic, spirits, souls, Elder Gods, Gods, Demons, and Devils exist in that universe. All four of them are essentially flesh golem-clones sustained by ancient magic.

The Conversion Bureau universe is essentially identical to our own universe: magic does not, and cannot exist. There are no soul, gods, spirits, demons or devils.

Irantu, Nanku, Munru and Onru step out of the portal, and drop to the broken, dirty plascrete road of the alley. They fall as if they were marionettes who just had their strings cut. They don't even twitch. They just flop onto the ground, four humanoid lumps of very dead meat wired up with cybernetic hardware. Their weapons skitter on the plascrete and stop. They stink, even in the constant smog.

The Blackmesh gradually stop staring, and one kicks at Nanku. The lump of meat jiggles like all meat does. The Blackmesh leader thinks to his helmet, and activates a channel through his bioport. He calls in the anomaly. Some of the Blackmesh wonder if the sudden arrival of four dead cyborgs has anything to do with the ongoing collision of two universes, others wonder if the Worldgovernment is experimenting with teleportation or something, they start talking about these ideas until the leader tells them to shut up.

They guard the bodies until they can be retrieved for study.

Full examination reveals only what appears to be almost a joke: four dead cyborgs, apparently from some kind of alternate earth, only - they could never have been alive. The whole thing is impossible. There is too much to worry about, what with trying to save 19 billion human lives from the end of the world, so the bodies are dumped in a scavenge pit, and the report is filed and forgotten.

Days later, Favela scavengers begin to rip bits off the bodies of Tau 5, looking for anything - clothing, permatech, implants, trinkets - that could be used for trade or for personal keeping.

The flesh of the bodies is gradually devoured by mutie-rats. The bones and useless metal parts of Tau-5 sink into the slimy oils at the bottom of the pit. When that patch of ground is finally taken by the Barrier and transformed into more of Equestria, it becomes a lovely pond, filled with shining fish and splashing frogs. Butterflies fly over it.

One day, four hundred thousand years in the future, a family of ponies, anciently descended from Newfoals, have a picnic beside that pond.

The End.

7610244
If unicorns can use their magic beyond the confines of Equestria, so can Samsara.

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Equestria doesn't actually have any magic - neither do unicorns.

Equestria is a machine. It is a 'Multiversal Mover', a type of multiverse-travelling ship built by a hyperdimensional species called the Krawlni. The 'decks' of the ship are pocket universes, layered on top of each other. The 'elevators' between decks are basically equivalent to TARDISes - time-space devices that can travel between the parallel universe 'decks' of the whole ship. Each 'deck' can be configured in any form - rather like the Holodeck on Star Trek... only, unlike the Holodeck, the configuration is real. It isn't an illusion. A Multiversal Mover deck could, for example, be created from a 'snapshot' of where you live on earth. It could be the entire earth, actually. That snapshot would not be a hologram - it would be real. A real copy of the entire earth, and that real copy would have real copies of every person, place and creature, and they would all be truly alive. They could be deleted or altered at whim - and they would literally be real people. The Krawlni do not have what humans would call 'morality'. They are deeply alien.

Equestria, in the Bureau stories I write, is a broken, crashed, smashed-up Multiversal Mover. It was wrecked on some natural phenomena that exist in the interstice between universes. It only has two 'decks' left, the rest have all been destroyed. It is barely functioning, and all the Kralwni on board died in the crash. Celestia, Luna and Discord were intelligent control systems within the Mover - Celestia was Environmental Control, Luna was Navigation, and Discord was the direct interface that the Krawlni used to control the ship. That is why he is so chaotic and strange - the Krawlni are more alien than Cthulhu... deeply strange indeeed. When the ship 'crashed', those systems were released from bondage and became free. Over time, they gradually took on the Pony identities we know, as they built a world on one of the remaining two decks. That world became Equestria.

What unicorns use as 'magic', isn't magic. It is high technology - there is no magic. This high technology uses alien physical laws, but it is still not magic. In effect, think of all the 'magic' in my stories as a Krell Machine granting unicorns 'tractor beams', 'teleport beams', and 'matter reconstruction' right out of any SF story. Pegasai can fly because they are being granted 'force fields' and 'gravity manipulation' from the machinery inside the Mover.

The Multiversal Mover that is Equestria, when it devours the earth, isn't just using all of that matter, space, time and energy to make more of Deck One: Equestria. A lot of that stuff is being used to refuel the Mover. Movers can refuel by absorbing chunks of spacetime from any universe: that is their 'gasoline', so to speak.

And that is why Samsara would drop dead. No magic. Just technology. Zero magic, zero souls, zero gods, zero mysticism.

In the stories, though, the characters don't know about this: they just call it magic. Because they are ignorant. At least, until the last novel, when everyone finally finds out what only a very few know.

"Any sufficiently advanced technology is indistinguishable from magic." - Arthur C. Clarke

7610518
How did these "Krawlni" Escape the wrath of the scarlet king?
They sound like a race he, or rather it, would be very intrested in.
Also, say the scp 5000 scenario ocured in the TCBverse, could it by any chance fix it?

They wouldn't die, they'd just reapear back in their home universe, with new bodies.

7610520

In the Bureau universe, the Krawlni are one of the most ancient of species in the local multiverse. The SCP universe is not part of that multiverse, but is in another multiverse entirely, part of the Local Melange along with many other multiverses, all within the Known Omniverse. Thus the Scarlet King is not an issue.

For anything to punch through from the earth that has the SCP foundation to the earth that has the Bureau history on it, would require hypermultiversal travel. They would have to punch outside their own cluster of universes, cross the Interstice, and then drill down into another cluster of universes, into the Bureau-containing array of Splays, and finally into one of the alternate earths that had a Conversion Bureau. It would be a hell of a trip!

7610041
No one cares because it doesn’t involve ponies. :D