The SEQUEL games are a series or ero RPGM games made by Hakika under "Leaf Geometry". There are 5 mainline games and 3 "spinoff" games. These are the 'modern', much more recent and polished games developed by Hakika. They all take place within the same world and follow eachother in a timeline. Aside from the spinoffs who get unique main characters, the mainline games follow the same protagonist (unfortunately a "blank slate" but ig something had to be sacrificed to get that OST)
"The Era of myth" games, as the community has come to call them are Hakikas first games that he made before rebooting his work with the SEQUEL games. Much like their younger brother these too take place in the same universe(s. Kind of. The worldbuilding goes a bit fucky. in a good way tho). Rather, it would be more accurate to say that the SEQUEL games take place in the dilapidated ruins of what was once the setting of each of the Era of Myth games. Most modern games take place in what remains hundreds or even thousands of years later.
If you wanna play the Leaf Geometry games then you can support the creator or not through DLSite (good luck with that though. i fucking hate american credit card companies). Regardless of how you've attained a copy, you're going to have to stop by a site like F95zone and download the game from there for the Fan translations of each. Aside from that the process is pretty smooth. The game order is SEQUEL Blight, SEQUEL Awake, SEQUEL Colony, ASLYLUM, SEQUEL Kludge, L.Depth, INOCENT RULES, and last for now; SEQUEL Thirst.
Id start with Blight first and just go through the cannon order.its a good jump point and inroduces the world the best imo. Maybe playing Asylum and Innocent Rules back to back could be a good idea as they directly lead into one-another and pair nicely together.
P.S. dont play the era of myth games, at least not before you've played everything else. Because god knows those are the first games hakika made and their story and gameplay certainly look and feel the part. Those games are genuinely a slog to get through and the main thing making it bearable was going like "HOLY SHIT *THATS* WHAT AN ERRA STONE IS" for 5 hours straight.
P.S.S. i wrote this at 4 AM so if anything is unclear/if you're having any problems please as more and id be glad to help in whatever way i can.