Chthon doesn't appear when I take the rune (or the quad, or the pentagram) on the fourth level. Is there something else I have to do to make him emerge? I can't progress until I kill him.

A very fast play. If it wasn't an episode it would just be a linear small map. Gameplay in the first three maps is firstly about eliminating the smallish hordes that attack you as soon as you spawn, and secondly on tracking down buttons that unlock the runs you need to complete the map. The first map is a little rough, giving you the ssg and nailgun vs. several death knights and company, but they get a bit better as they introduce more weapons and monster variety. Unfortunately none of the maps introduce more monsters after the initial big fights, so they pretty front-loaded.

Fourth map is a battle vs. a modified Chthon who can be damaged with your weapons. He is supposed to spawn when you grab the rune, but you're not missing much if he doesn't since fighting him is all there is to the map, and the last map is just a small room with an exit teleporter and a shambler that spawns in front of it.

I mainly played this for the ikblue textures, but these maps don't look as good as Iikka Keränen's. They're a little bit of fun though, so 3 stars is about right.