The latest episode of The Flash drops a reference to the first major Arrowverse crossover, "Invasion!". In 2016, the Arrowverse had its third crossover event, but this one was the biggest yet, as the first two only included The Flash and Arrow. "Invasion!" brought together The Flash, Arrow, and Legends of Tomorrow. While Supergirl was not incorporated, its titular protagonist did serve as one of the event's main characters.
In The Flash season 6, episode 4, titled "There Will Be Blood," Barry (Grant Gustin) convinces Cisco (Carlos Valdez) to help him find a cure for Ramsey (Sendil Ramamurthy). The two make a deal with the newest version of Harrison Wells, Nash Wells (Tom Cavanagh), who tells them of the existence of a rare bioregenerative serum capable of curing anything, including cancer. After retrieving the serum, Barry gives it Ramsey. Unfortunately, it doesn't work, and Ramsey finds his own solution for what's ailing him. He goes on a killing spree and becomes the Flash's next big villain.
Where did this miracle serum come from? According to Nash, it was appropriated by McCulloch Technologies during "some invasion" by the alien species known as the Dominators. He's of course referring to the time the Dominators came to Earth in 2016. The Dominators were the villains of "Invasion!". Apparently, some people on Earth were able to reap certain benefits from the Dominators' attack by stealing their technology.
The reference is an interesting way to tie in to the first Arrowverse crossover story that united Barry, Oliver (Stephen Amell), Kara (Melissa Benoist), and the Legends. Barry and Kara already knew each other, but this was what fully integrated her into the world. It was also the first time that the fully-formed Legends were involved in a crossover, although only a few of the characters were involved in the 2015 event, since it was designed mostly to set up Legends of Tomorrow.
"Invasion!" was a huge moment for the Arrowverse. The sheer number of superheroes included in this three-part event was unprecedented for a live-action DC Comics show. However, what "Invasion!" was able to do with its characters feels almost trivial when comparing it to what's about to unfold. The 2019-2020 crossover, "Crisis on Infinite Earths", is bigger than anything that's come before it, as it will encompass all five Arrowverse shows, along with several other shows like Smallville that were never even part of the Arrowverse, but the reference to "Invasion!" on The Flash is a nice way of recognizing its importance and how far things have come since.