Forge version: 1.12.2 - 14.23.5.2768
Twilight Forest version: 1.12.2 - 3.8.689

Steps to reproduce:

1. [Green Particles] Just walk up to either a boss or a random place in the world and random green particles starts to appear out of the bloom.
2. [Random Black and Red Particles] Happens randomly when you go into a different biome(AKA you are basically screwed in the end)

What I expected to happen: No particles covering my screen and couldn't see barely anything

What happened instead: A ton of particles covering my screen and couldn't see barely anything

Pictures/Screenshots:

Check your Advancements. Have you unlocked any of those areas yet?

What do you mean by that?

This page explains it all, but in short, Biomes and Structures are locked behind a Progression barrier. You have to go through each boss in order, as seen in Advancements, before you can enter the next area. You can disable the barrier with /gamerule tfEnforcedProgression false.

Oh okay that makes a bit more sense, also I tried the command you gave me and I still get the particles.

You may need to leave the Twilight Forest/the save file then enter back in if you use the command (#627). It's just a visual bug.

Okay, also while fighting some of the monsters, the giant Naga escaped from their building and the game won't let me hurt it, is that supposed to happen or...?

It's intended. The Naga is supposed to return to the Courtyard if it ends up outside of the boundaries, but at the same time it becomes invulnerable until it is back inside.

Going to assume this is resolved.