Shader compilation log for iris:sodium-terrain-shadow.vsh: 0(39) : error C1503: undefined variable "Chunks"
https://mclo.gs/bMCcc8U

Should be fixed in 1.6.6.

Replicated the issue fairly easily with Renderpearl 1.7 (https://mclo.gs/EeN9Che) and confirmed it with a dev build of my own shaderpack (https://mclo.gs/IWFCiG0).

I think the problem arises when a shader declares the varying vaPosition with in vec3 vaPosition; - glsl-transformer replaces any references to it with Chunks[_draw_id].offset.xyz + _vert_position but doesn't declare Chunks anywhere (and for that matter, doesn't remove the vaPosition declaration).

Already fixed in dev branches.