Complex sentence: Buffy explains that Spike wasn't invited because he wouldn't behave around her friends, however, having heard about the party from Willow, Spike shows up with beer and Clem, the loose skinned demon with whom he plays kitten poker.
Split into simpler sentences: Buffy explains that Spike wasn't invited because he wouldn't behave around her friends. However, having heard about the party from Willow, Spike shows up with beer and Clem, the loose-skinned demon with whom he plays kitten poker.