Iris Warning: Unknown variable: BIOME_SWAMP_HILLS (Not the Actual Crash)

What it looks like in the log

java.lang.RuntimeException: Unknown variable: BIOME_SWAMP_HILLS in Prism Launcher crash

How to fix it

1. Check the prefix on the error line in latest.log: because it says [Render thread/WARN], ignore this stack trace and look elsewhere for the real crash cause.
2. Open your instance folder (in Prism Launcher, select the instance and click Folder) and check the crash-reports directory for a file matching the exact time the game closed.
3. If no new file exists in crash-reports, look directly inside the minecraft (or .minecraft) folder for a file named hs_err_pidXXXX.log, which contains the real error when Java or your GPU driver crashes natively.
4. Press K in-game (default Iris keybind) to toggle shaders off temporarily and test whether the random crashes stop, which helps isolate GPU timeout or driver issues.
5. Update your shader pack to a newer release built for Minecraft 1.20+ if you want to remove the BIOME_SWAMP_HILLS warning from your logs.

Check your own log