Where to start the series?

[Shadow]

I want to start the series and i know the first game is a H-game that isn't on Steam so i want to know the game order after the first game and is there any missing game that isn't on Steam. Also i saw a review say that Window Anti Virus detect this game as a threat is it still a thing?

[Reyes]

The first game is on Steam, it is Prelude to the Fallen, the original PC version was +18 but Prelude is a version that came out on PS4 without that content and was later ported to PC

[Minneyar]

Prelude to the Fallen is an expanded remake of the original 2002 H-game and is on Steam, so that's the best place to start. Play order for the main games in the series is:
Prelude to the Fallen -> Mask of Deception -> Mask of Truth -> Monochrome Mobius

I don't know if Microsoft Defender still detects them, but it was always a false positive, you can just ignore it if it does.

There are a few spinoff games that are not available on PC, but they don't really add anything to the main story, and most of them aren't in English. The only one that's in English is Utawarerumono Zan, an action game for the PS4 that is based on a highly abridged version of Mask of Deception. In Japanese, there's also Zan 2 (same thing but based on Mask of Truth); Dokapon UP!, a board game on the PS4/Switch with Utawarerumono characters; and Ginzoku Tantei Nosuri, a short comedy/mystery VN on the PS4/Switch.