Fright Night: Darksynth is a set of Serum synth sounds, with excellent sound quality and useful presets. They have an analogue essence to them that are not often found in many preset packs. And the Cthulhu set is a wonderful, useful bonus. I have also used Glitchedtones' Retrowave (Synthwave) presets and immediately liked their talent in creating sound sets. I highly recommend Darksynth for soundtrack and darker forms of electronic dance music in general.
Full Disclosure: I initially had minor difficulties with my order, but the team at Glitchedtones quickly rectified the situation. I could sense they take their work seriously, which only makes me more interested in their other sound libraries.