Is there an existing issue for this?
I have searched the existing issues and not found any

Observed behavior:
I have been able to run Minecraft for quiet sometime on v1.2.0.
After updating to v1.3.1 (no other mod changes), the Client crashes during startup.

Ice and Fire RotN version: IceAndFireRotN-1.9.1-1.3.1
Updated from: IceAndFireRotN-1.9.1-1.2.0

Forge version
1.12.2-forge-14.23.5.2860

Other mods
llibrary-1.7.20-1.12.2
RLMixins-1.2.19

Below you can see the involved logs:

• Crash Log
• Debug Log

Results:
I was able to determine the specific incompatibility:

RL Mixins is currently incompatible with:

• Ice and Fire: RotN Edition after v1.2.0

RL Mixins is compatible with:

• Ice and Fire: RotN Edition v1.2.0 (released 5/2022)

as long as the following RL Mixins configs are set to false:

• Enable Better Dragon Bites (IceAndFire)
• InF Offhand Items (IceAndFire)

This fork fixes many bugs with the original Ice and Fire mod that RL Mixins also tries to fix, causing the crash you experienced. Because RL Mixins is specifically designed for RLCraft and this fork has no intention of using RL Mixins, this incompatibility is not something that will be fixed.

If you set these RL Mixins configs to false:

• Lycanite Targetting (LycanitesMobs/IceAndFire)
• Stoned Chicken Feather Fix (Quark/IceAndFire)

then RL Mixins is fully compatible with the latest version of Ice and Fire:

• Ice and Fire: RotN Edition v1.9.1-1.3.2

Thank you so much to the developer for the amazing work.