The final episode of Ben 10 OS was set in a parallel universe as well.
Z'Skayer (AKA Ghostfreak) I honestly no idea on how he managed to escape, all I know is that something happened but I have no idea on the how or the why.
To clarify: I try to avoid Ultimate Alien at all costs because of the fact that "Ben is now a widely-known celebrity on Earth (which would seemingly defeat the purpose of First Contact with alien life since the entire planet now knows about it more-so than ever before), adding to it the popular media of the galaxy saying that Ben saved the entire universe from the Highbreed (come on, the universe is at least 13 billion years old, and would take a seemingly large amount of time for one to traverse barely an inch of that if it was on a map that was only 13 feet in length and diameter. Even if it were possible, there are still hidden areas, uncharted corners of space, unknown lifeforms hiding within our own galactic backyard that refuse to even be found by anyone. Plus again, the Highbreed are the most advanced alien threat to have ever been seen that had the largest scope of power at the time. If this was the case than their only targets for genocide should have been only in the immediate area of their own galaxy plus the neighboring Magellans if there is life out there which I firmly believe to be true –god I hope there will be some cool alien cultures in the Magellans, or at least some cool backstory about life there that’d influence why the milky way is like the way it is right now instead of being more like in Star Trek with scientific pursuits or cultural disputes instead of Americanized bullshit like we’ve seen with a known interstellar pair of southern US stereotypes)
To clarify: there is a group of like-minded individuals that also hate the turn of the series with Omniverse, just go to the group Ben 10 OS-to-UA for more details and debacles on why it is terrible and about quality artwork by fans like you.