which third annual Arrowverse crossover event did "Medusa"  features 
Medusa (Supergirl):  The episode features a minor tie-in with the Arrowverse crossover event "Invasion! Invasion! (Arrowverse): "Invasion!" Invasion! (Arrowverse):  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! Invasion! (Arrowverse):  Each of the latter three episodes were titled "Invasion!" Invasion! (Arrowverse):  "Invasion!"
"Invasion!"