The Arrow season 8 premiere rewrote season 1 by killing off a key original character - Thea Queen (Willa Holland) - resulting in a major ripple effect for the timeline of Earth-2's Starling City. The CW's flagship superhero series kicked off its final season by building towards this year's mega-crossover, Crisis On Infinite Earths. At the end of Arrow season 7, Oliver Queen (Stephen Amell) left his family and friends behind to join the Monitor (LaMonica Garrett) on a mysterious quest involving the imminent Crisis.
In Arrow's season 8 premiere, "Starling City", Oliver arrived on Earth-2 and found that his city in 2019 was like a funhouse mirror image of Arrow season 1. The Green Arrow was rescued from the island of Lian Yu after 12 years (not 5) and returned home to find people who are dead in his reality alive and well - but not everyone. His mother Moira (Susannah Thompson) is now married to Malcolm Merlyn (John Barrowman) and his best friend Tommy Merlyn (Colin Donnell) is alive, plus the reformed former Black Siren, Laurel Lance (Katie Cassidy), is now Earth-2's Black Canary fighting alongside a new Hood, Adrian Chase (Josh Segarra).
However, in Earth-2's reality, Thea Queen overdosed on the narcotic Vertigo and died on her 18th birthday. This set off a chain reaction of events from Arrow season 1 playing out very differently. For one thing, the Undertaking - which was originally Malcolm Merlyn's supervillain plan to destroy the Glades with an earthquake machine - was now planned to occur six years later. This time, the Undertaking is Tommy's scheme to wipe out the Glades; the younger Merlyn is the Dark Archer on Earth-2 and his motivation to kill thousands of people is centered around losing Thea. As Thea's half-brother, Tommy was distraught by her death and he blamed the crime in the Glades for getting Thea hooked on Vertigo; the Earth-2 Undertaking is Tommy's revenge for Thea.
On Earth-1, Thea did overdose on Vertigo on her 18th birthday (in the Arrow season 1 episode "Trust But Verify") but she got in a car crash, which she survived. Thea went on to have a heroic arc of her own; she discovered Malcolm Merlyn was her true father and he trained her in the martial arts of the League of Assassins. Thea eventually joined Team Arrow and became a costumed archer named Speedy; she also became Oliver's Chief of Staff when her big brother was elected Star City's Mayor. However, after a difficult life that includes being rescued from near-death thanks to the Lazarus Pit, Thea eventually left the madness of Star City behind to build a new life for herself with her boyfriend Roy Harper (Colton Haynes). And, although Thea has been dead for years on Earth-2, Willa Holland will reprise her role for Arrow's final season.
Arrow's season 8 premiere was filled with even more callbacks to Arrow season 1. Oliver discovered Earth-1's John Diggle (David Ramsey) followed him to Earth-2 and posed as his bodyguard, just like he did when they originally met and Ollie even used the same tricks to try to evade Dig's shadow. Oliver's return to his family's mansion played out almost identically to the events in Arrow's pilot, only Tommy was in Thea's place and Malcolm substituted for Moira's second husband on Earth-1, Walter Steele (Colin Salmon). Of course, Tommy's plan to level the Glades was similar to his father's on Earth-1, only the younger Merlyn utilized dwarf star technology instead of an earthquake machine; the Dark Archer's rooftop fight with Green Arrow is similar to Oliver's fight with Malcolm in the Arrow season 1 finale.
Given that the events of "Starling City" happened years later then they did on Earth-1, there were some notable changes, like Black Canary having her own underground bunker headquarters, which was inspired by Oliver's and Laurel's time working with Team Arrow on Earth-1. Tragically, the Arrow season 8 premiere ended with the total destruction of Earth-2 by an antimatter wave, which is the harbinger for the Multiversal doom imminent in Crisis On Infinite Earths.