The Arrowverse's 'Crisis on Infinite Earths' may substitute the Antimatter Cannon for an Antimatter Arrow
After finding out that the Monitor / Mar Novu (LaMonica Garrett) may actually be the cause of the apocalypse and not the cure, Oliver Queen / Green Arrow (Stephen Amell) is dead set on creating a weapon to take out the cosmic being.
As of Episode 4 'Present Tense', he has enlisted the services of Curtis Holt / Mr. Terrific (Echo Kellum) to use the same strange energy that destroyed Earth-2 to build a Monitor-killing weapon, however, there may be more forces at work here than Oliver is aware of.
'The Flash' has confirmed that the blood-red wave that destroyed Earth-2 was made of antimatter, which suggests that Oliver's weapon could be a variation on the Anti-Monitor's Antimatter Cannon from the comic book 'Crisis on Infinite Earths'.
The Antimatter Cannon was a device created by the Anti-Monitor, who is also being played by LaMonica Garrett in the Arrowverse, as a weapon that can destroy the entire multiverse.
What Oliver has in mind is of much smaller scope. He just wants a weapon that can kill the Monitor so perhaps we may be getting an Antimatter Arrow instead of a cannon, which would honestly look really cool.
It's a cardinal rule in storytelling that a weapon once introduced must be fired at least once. Even if Oliver doesn't end up killing the Monitor, there's still the chance that he could be playing into the hands of the Anti-Monitor, who would be able to use Oliver's antimatter weapon to further his own plans of multiversal destruction.
If Oliver is unwittingly aiding the Anti-Monitor's cause by creating a cosmic WMD, then the last scene of the episode where the Monitor asks Laurel Lance (Katie Cassidy) to betray Oliver makes some sense.
However, if the Monitor really is the destroyer that Oliver thinks he is, then things are about to get a lot more complicated. 'Arrow' Season 8 Episode 5 'Prochnost' will air on The CW on November 19.