So, a bug in Act 3's Universal Truth. When you are told to Run...
The Great Darkness either doesn't spawn in, or does and is invisible (missing textures etc), or can't be seen with the baseline recommended lighting settings and an ambient factor of 120 and no deferred lighting.
It really takes a lot of the impact out. Build is: SSE 20151103_0ea25e.
Command line: -mod
bpcomplete,mediavps_2014 -nomotiondebris -missile_lighting -3dshockwave -post_process -soft_particles -fxaa
-nolightshafts -fb_explosions -no_deferred -disable_shadows -snd_preload -ambient_factor 120 -spec_exp 11
-spec_point 0.6 -spec_static 0.8 -spec_tube 0.4 -ogl_spec 60