I've always loved the rules. There's 66 rules I believe, or at least parts of how to use the Death Note. The first one (this one) and the last are my favorite. The last simply says that any human who has used a Death Note goes to neither heaven nor hell when they die. All humans must die at some point, and those who have used a Death Note will never come back to life. There's so many rules and technicalities, I love it for this concept.
The Shinigami realm is what I'm thinking of. It's almost poetic. They had their own realm, their own world, and they killed everything there with their death notes. Now they're left on a desolate rock doing the same things everyday. Trying to stave off boredom. They have nothing left and nothing to do but gamble. Sure they can write a humans name in their Death Notes for the sport of killing a human, but they take very little joy in that anymore. As gods of death, it's not even a challenge.
Aaaand this thing. Clever.