tywyll - dark as in nos dywyll ('a dark night'). tywyllwch - darkness.
gwyll - dusk, gloom, twilight. This is probably what is inferred in the drama's title - the content is constantly referring to past mis-deeds and hints at shadowy goings-on, etc
The GPC (a Welsh etymological and historical dictionary) mentions the possibility of a shared element in the origins of tywyll and gwyll.