Summon Night is the starting weapon for Syuuto and can be unlocked for all by spending 15+ minutes as Syuuto.
Syuuto is unlocked by evolving the Four Seasons.
See Vampire Survivor wiki for further info if required.
https://vampire-survivors.fandom.com/wiki/Four_SeasonsOnce you have Summon Night, you will need to get this to max level (8) and also get Duplicator to max level (2).
Once both are fully upgraded, open a chest which is dropped by the bosses to evolve Summon Night into Echo Night.
I used the Randomazzo card “VIII - Mad Groove” on the new level Mt. Moonspell. This has a duplicator spawn on the map and the card brings everything to your location every 2 minutes.