Game immediately crashes when loading up minecraft 1.12.2, on forge v14.23.5.2859, this is the mod list:

TheBetweenlands-3.9.6-universal.jar
Alfheim-1.0.2.jar
!Red-Core-MC-1.7-1.12-0.5-Dev-3.jar

Specification:
OS: Windows 10
CPU: Ryzen 5 5500
GPU: Radeon RX 5500 XT
RAM Allocated: 8GB
No crash report is generated by the game.

Crash also happens with Hesperus, but not with the original Phosphor

This is almost certainly due to The Betweenlands's shaders, try disabling them (if necessary, through its config file directly) and try again. I'll look into this a bit more when I have time and will report here if I find anything.

Apparently I was missing a dependency for that mod, MixinBooter, and enabling it fixed the issue.