It's called "Vasari".

No.

I sure hope not. The way they're described, they'd trounce everything. There wouldn't be any point.

Oh no!

It's the space ponies!

It'd make for one hell of a challenge... Perhaps too much of a challenge

Nanites is still what I'm voting for, but yeah, their ships would probably consist solely of particle effects grouped into the following:

Assault Cloud: large, moderate health, dissolves other ships nearby, capable of creating colony clouds for a health sacrifice.

Colony Cloud: small, low health, "colonizes" (aka turns to grey goo) planets.  All colonized planets cannot be colonized by the enemy.

All ships hit by the DoT that die as a result create assault clouds but must wait a duration of sixty seconds before becoming operational.  Another note is that all attacks made by the nanites heal them.  Clouds have no auto-repair.  Due to a lack of complex construction, 

In other words, more or less unstoppable.  Massed Kostura Cannons is honestly probably the only thing that'll pound them out of existence.  Actually!  That gives us an explanation as to why the Kostura cannon was ever built in the first place.  It creates a phase space node and destroys things in orbit.  Fire behind you to destroy the plague, fire ahead to run.

I know it's outstandingly unlikely, but if we were given the ability to mod tutorials (aka make campaigns), I would personally love to make a campaign with the last few missions consisting of discovering their presence and proceeding to attempt to outrun them as, say the TEC.  Once you do, your next objective is to meet up with envoys from the Advent and Vasari.  Doing so completes the mission.  The next and final mission of the campaign is making a last stand against them with the combined effort of TEC, Advent and Vasari Militaries.  Since these things would be immune to culture, the advent instead must use shields to guard the TEC ships as they defend the Vasari long enough to mass produce Kosturas before a final bombardment ensues in which all plague are wiped out.

If you ask me, great way to end the game.  It's sort of a high note in the diplomacy of the races and means that they could possibly coexist with at least a cease fire in place.  Good place to end the game, or if in Sins II or III what have you, the franchise.  Point is, whenever IC decides no more Sins, that's the time to put these campaign missions.

That would certainly be the most "diplomatic" solution to things, but everything living ever after in peace and harmony would not make sense given what we know (that advent are quite bloodthirsty, that Vasari had a tendency to be harsh, overbearing rulers.)

It would also destroy the premise for the 3 races fighting each other.

Also, I would think that the Vasari probably already did all they could to combat the unknown menace when they actually had the resources of their entire empire 10,000 years ago, when they sent their huge fleet there: right now, they're relatively (resource) poor migrants.

in regards to it destroying the will of the races to fight each other, that's why i said put it at the end of the franchise.  Whenever IC decides its time to kill Sins as a franchise, that's when it's time to put that campaign mission in.

in regards to the Vasari having already done everything, I'm saying that they developed the Kostura on the run.  Think about it.  Do you honestly think the Antorak Marauder would have been built before or after Kron's mishap?  The correct answer would be after.  It's purely for running away and for helping other ships do the same.  What about the Kostura?  What does it do?  Pound all ships in orbit.  What else does it do?  Create a phase node.  Fire ahead of you and run away via your Antorak.  Fire behind to slow the spread of the Darkness

I think that all three superweapons should be able to affect it if this was done, assuming it is gray goo, the Novalinth can nuke it out of existence and the deliverance engine can jam the signals holding the swarms together.

Alternatively, it could be made that you could play the campaign as any of the races, over the missions you acquire technologies from the other two races, culminating in both of them giving you their superweapon techs before the last mission, which your scientists combine to make a superuberweapon that nukes a planet, disables/destroys structures/ships in orbit, creates a phase node (supposing Vasari phase nodes can be acquired by the other races), and spreads your culture (supposing the Advent's culture vision can be acquired by the other races.)

In the ending scenarios, you fight delaying actions against the Darkness, trying to stay alive for X amount of time or hold a choke-point planet for so long. The other two races' high commands approve ceasefires during these missions, joining you and allowing you to concentrate on slowing the enemy's advance. Meanwhile, in the core worlds of each faction, scientists race to find a way to combine the Novalinth, Kostura, and Deliverance as I stated above. You are dispatched, as your chosen factions best sector commander, to try to support your allies' core planets but both are overrun despite your best efforts (these are two more delaying action scenarios.)

The survivors are evacuated to your home sector where their scientists combine their efforts with yours as all three fleets prepare for a desperate defense of their last hope of salvation (the last scenario starts with all planets divided evenly between the three factions, all but one of the research items are finished, this tech is called different things for each faction, but all three unlock the superuberweapon, it is only available to human players and is extremely expensive, and takes a half hour to complete.)

The opening attacks of the Darkness are on your allies' planets, when it reaches a pirate base, a cinematic shows the base being captured. A message comes from the pirate factions (who have been conducting their usual raids) pledging allegiance to your empire while the pirates appear on the diplomacy screen as "Allied" with you and both visions. The remaining pirate bases on the map are emptied of ships as the pirate fleets move to bolster the crumbling defensive forces of your allies.

As the darkness breaks through your allies and starts assaulting your worlds, you finish research for the weapon, which is constructed like a starbase. You build some and start blasting away at infected worlds. As they are cleansed, you move in weapon constructors to increase your firepower, of course, you must move ships up with them, because once a Novalinth-esque timer expires, they are able to be recolonized by the Darkness (and you can't colonize any planet that has been under Darkness control). The end objective of the mission is to defeat the Darkness. An ending movie shows your empire establishing a defensive perimeter to hold back the Darkness, and destroying/subjugating the remnants of the other two (I don't share your optimistic hope that the shared enemy will cause them to permanently ally, plus, if your going to make a franchise killer, make it so you can't do what Pirates of the Caribbean did and try to keep going)

dude. homeworld cataclysm??? remember "THE BEAST" i think THAT is what it should be!