SPOILER WARNING: This article contains major spoilers for "Orientation," the Season 5 premiere of Agents of S.H.I.E.L.D.
It is no accident that the first episode of the latest season of Marvel's Agents of SHIELD is called "Orientation," as the viewers spend most of the episode in the dark about what going on with the agents and the viewers learn about what is going on just as soon as the characters literally re-orient themselves into their new home in outer space and discover the two major aliens that the heroes have to deal with this season. The first is the Kree, who have been featured in previous seasons on Agents of SHIELD and who also showed up in Guardians of the Galaxy as the main villain. The Kree are likely one of the two or three most commonly featured alien races in Marvel's comic book history (only the Skrulls and the Shi'ar could possibly match the Kree in terms of comic book prominence). The other race, though, the Vrellnexian, are likely one of the most obscure alien races in Marvel Comics history. Read on to learn more about the comic book history of this race and how they tie into the new season of Agents of SHIELD.
The big twist in the Season 5 premiere is not just that Agent Coulson and his crew have transported into outer space and are on a ship where they are jailed by the Kree, but the fact that all of this takes place many decades into the future! In the future, the Earth has been devastated, with most of the population killed. It was here, then, that the Kree stepped in. One character, Tess, reminded the others of what happened in the premiere, "All we really know is the planet was torn apart. Some cataclysmic event. Then the Blues showed up, restored order, but destroyed all historical data — files, books — so it would keep us focused on the future. Now they oversee everything that’s left.”
This is where the Vrellnexians stepped in. While humans could no longer inhabit the Earth, the Vrellnexians could and they have taken up shop on Earth's surface and they have also ended up hunting down the humans who survive in the Kree space station above Earth. They're colloquially referred to as "roaches" by the surviving humans.
While there are certainly some variations, this is very much how the Vrellnexians appeared during their first (and only major) comic book appearance back in 1973's Thor #212 (by Gerry Conway, John Buscema, Don Perlin and Vince Colletta). At this point in the comic book, Odin had been on one of his infamous capricious mood swings and had banished Thor and a number of warriors from Asgard for one reason or another. However, the banished warriors learned that Asgard was in peril and so, of course, they hurried to the aid of their home, even while in exile. Once there, they found it inhabited by a lizard-like race of aliens. As it turned out, the lizard-like aliens had been the victims of the Vrellnexian, an anthropod-like alien race that conquers planets when they are at their weakest point and then enslaves their people and sells them into slavery. The lizard people were just being held on Asgard after the Vrellnexians had conquered Asgard in the absence of its greatest warriors...
Never one to let a grudge get in the way of helping out his fellow Asgardians, Thor and his friends traveled to an intergalactic auction plot and rescued Odin from the auction block and defeated the Vrellnexians...
Of course, this being comic books, it turns out that the lizard people were just using Thor to take out the Vrellnexians so that they could take over as the head bad guys (naturally, Thor ended up defeating them, as well).
That story really was, oddly enough, the last major storyline to feature the Vrellnexians. The next time they showed up, they were being studied in the pages of Quasar #49, a comic book series written by the late, great Mark Gruenwald, who was famous for his vast knowledge of Marvel Continuity, so leave it to someone like Gruenwald to have characters study such an obscure race. In that issue, the character Epoch explained the Vrellnexian paradigm, which is that "every single action by every single organism causes reality to diverge within its own phenomenal field creating interference patterns between adjoining fields."
This makes sense, as the Vrellnexians are very much a hive mind creature, so it is sort of like a massive version of the Buttefly effect (Gruenwald was also a major proponent of the Marvel time travel theory that every change made to time did not actually change time itself, but rather caused a divergent timeline in the Multiverse).
The Vrellnexians have mostly held a background role in the Marvel Universe since then. They were one of the collection of alien races who got together during Maximum Security (by Kurt Busiek and Jerry Ordway) to determine that something be done about Earth (the solution of the collective of alien races was to turn Earth into a prison planet so that they could stop having to worry about the beings from Earth causing trouble in the rest of the galaxy).
It is worth noting that with Agents of SHIELD being set in the future now, that the Vrellnexians have made appearances in the future before, in the 2099: Manifest Destiny one-shot, where we got to see the future of the Marvel Universe after the year 2099, in a sort of wrap-up to the continuity of the Marvel 2099 universe in 1998. We saw the Vrellnexians in the year 2399, where they were one of the many alien races who had noted that they had had trouble with humanity in the past.
All in all, the use of the Vrellnexians goes to show just how useful the wide tapestry of the Marvel Universe can be when it comes to picking out cool obscure characters to use (and, as Arrow has shown us with Felicity Smoak, great things can happen when you use obscure old Gerry Conway comic book creations).