Question: which third annual Arrowverse crossover event did "Medusa"  features 

Context paragraphs:
Medusa (Supergirl): "Medusa" is the eighth episode of the second season from The CW television series "Supergirl", which aired on November 21, 2016.  The episode features a minor tie-in with the Arrowverse crossover event "Invasion! "

Night of the Hurricane: Night of the Hurricane is a one-off programming block that introduced the first crossover event on the "Animation Domination" lineup on Fox.  The block involved the three animated television series created by Seth MacFarlane: "Family Guy", "American Dad! ", and "The Cleveland Show".  The event depicts a hurricane which hits the towns of Stoolbend ("The Cleveland Show" setting), Quahog ("Family Guy" setting) and Langley Falls ("American Dad!"  setting).  The actual three-way crossover of the block occurs at the end on "American Dad!"  with the three fathers of each family in the same scene.

Avengers Disassembled: "Avengers Disassembled" is a crossover event involving the Avengers, Fantastic Four, Captain America, Spider-Man and Thor.  "Avengers Disassembled" saw the beginning of Brian Michael Bendis' Avengers run with the destruction of the existing "traditional" roster and exile of several key members of the team.  The other crossover stories intersect (or in the case of the Spectacular Spider-Man and Thor crossover issues) or take place prior or after to the main Avengers story-line.  In particular, the tie-ins saw other changes to the status quo: Iron Man once again closeting his secret identity, Spider-Man developing organic webshooters (like the Sony Spider-Man movies), and the death of Thor and Asgard in one final Ragnarok.

Death of X: "Death of X" is a 2016 crossover miniseries published by Marvel Comics.  "Death of X" is part of a series of story arcs that have impacted the Inhumans and X-Men titles after Secret Wars ended and Earth 616 was restored along with being related to the events of the Infinity, Inhumanity, and Inhuman story arcs.  This miniseries event answers the question about what really happened between Cyclops and the Inhumans in the eight-month gap between "Secret Wars" and the All New, All Different lineup of comics.  This limited series event serves as a prequel to the series "Uncanny Inhumans", "Extraordinary X-Men", and Volume 4 of "Uncanny X-Men", along with impacting the plot to "Civil War II: X-Men" and ultimately serving as a precursor to lead into the major crossover event "Inhumans vs. X-Men".

List of Marvel Cinematic Universe film actors: The Marvel Cinematic Universe is a media franchise and shared fictional universe that is the setting of superhero films independently produced by Marvel Studios, based on characters that appear in Marvel Comics publications.  of the franchise includes five films, featuring four different superhero properties, leading up to a crossover in the 2012 film "Marvel's The Avengers".  The franchise's Phase Two features three sequel films to properties that appeared in Phase One, as well as two new film properties, and the crossover "", which released in 2015.  Phase Three will feature four sequel films to previous properties, and four new film properties, as well as the two-part crossover event "" and its untitled sequel.

Crisis on Earth-X: "Crisis on Earth-X" is the fourth annual Arrowverse crossover event, crossing over episodes of the live-action television series "Supergirl", "Arrow", "The Flash", and "Legends of Tomorrow" on The CW.  The crossover will begin on November 27, 2017 with "Supergirl" and "Arrow", and conclude on November 28 with "The Flash" and "Legends of Tomorrow".  "Crisis on Earth-X" sees Barry Allen and Iris West's friends come to Central City for their wedding, only to be interrupted when villains from Earth-X disrupt the proceedings.

Invasion! (Arrowverse): "Invasion!"  is the third annual Arrowverse crossover event, crossing over episodes of the Arrowverse television series "The Flash", "Arrow" and "Legends of Tomorrow" on The CW, and is inspired by the 1989 comic miniseries "Invasion! ".  Events at the end of the "Supergirl" episode "Medusa" on November 28, 2016 marked the beginning of the crossover, which officially began on November 29 with "The Flash", continued on "Arrow" on November 30, and concluded on "Legends of Tomorrow" on December 1.  Each of the latter three episodes were titled "Invasion!" .  "Invasion!"  sees Barry Allen recruit Kara Danvers / Supergirl to his Earth to help his team, Oliver Queen and his team, and the Legends to defeat the alien race known as the Dominators.

Flash vs. Arrow: "Flash vs. Arrow" is the first annual Arrowverse crossover event, broadcast on The CW, featuring episodes of the Arrowverse television series "The Flash" and "Arrow".  The event began on December 2, 2014, with "The Flash" episode "Flash vs. Arrow" and concluded the next day with the "Arrow" episode "The Brave and The Bold".  The crossover sees Team Flash (Barry Allen / Flash, Caitlin Snow, and Cisco Ramon) helping Team Arrow (Oliver Queen / Arrow, Felicity Smoak, and John Diggle) take on the boomerang-wielding villain Captain Boomerang, while Team Arrow helps Team Flash confront the metahuman Roy Bivolo.

Heroes Join Forces: "Heroes Join Forces" is the second annual Arrowverse crossover event, featuring episodes of the television series "The Flash" and "Arrow" on The CW.  The event began on December 1, 2015, with "The Flash" episode "Legends of Today" and concluded the following day with the "Arrow" episode "Legends of Yesterday".  The episodes feature characters from "DC's Legends of Tomorrow", which was being developed as a spin-off.  The crossover sees Barry Allen / Flash and Oliver Queen / Arrow team up to take on Vandal Savage, who is looking for Kendra Saunders and Carter Hall, the reincarnations of Hawkgirl and Hawkman.

List of Amalgam Comics publications: Near the end of the "DC vs. Marvel" crossover event in 1996, Amalgam Comics released a series of one-shot comic books combining characters from the Marvel Universe with characters from the DC Universe.  The first twelve Amalgam titles were released in a single week, temporarily replacing both publishers' regular releases.  Half the comics in the event were published by Marvel Comics and half by DC Comics.  A year later, the stunt was repeated, but without the crossover as background.  Later, both publishers collected their issues into trade paperback collections.

Supporting paragraphs: Medusa (Supergirl) Invasion! (Arrowverse)
Answer: "Invasion!"