One common theme is power struggles. In a dark harem setting in the Harry Potter universe, characters often vie for power, whether it's magical power or influence within their group. For example, a character might be trying to gain control over the dark arts while also trying to maintain relationships within the harem.
Love and relationships are also a big theme. It's not just about romantic love, but also about the bonds between friends and allies. In a dark harem, these relationships can be twisted. For instance, a character may love someone but also be in competition with them for power or status. And the different personalities within the harem can lead to all sorts of relationship dynamics.
Another theme is loyalty and betrayal. Since there are multiple characters in a harem - like situation, loyalty can be a complex issue. Some characters may seem loyal at first but then betray the main character or the group for their own gain. This adds drama and tension to the story, like in many fanfictions where a character from Slytherin might betray the group for a chance at more power.