I've seen a version of this challenge for the Sims 4, but Since I refuse to give EA any more money, I think I'll reverse-engineer the challenge for the games I do have.
-Make two sims (or two families, depending on how you want to challenge yourself) a "light" family and a "dark" family. The "light" family is your typical, ordinary sims family. They have jobs, the kids go to school if there are any, and basically, do not function any differently than any other sim family in the game. The "Dark" family, however, is a bit different. All of the sims in the Dark family are YA or older, and all have the "hates the outdoors" trait, as they quite literally never see the light of day. They live in the walls (or attic or basement, depending on how you construct the lot) of the Light Family's house and siphon all their needs off of the Light family like a parasite.
- Move the combined families onto an empty lot. Before anyone has a chance to talk to each other, QUICKLY get the members of the dark family into a secure place on the lot where they cannot see or be seen by other sims. Then, construct the house of the "light" family and get them settled. once the light family is securely inside, build a "house behind the walls" for the dark family, and delete the access to the outside once the dark family is securely in there.
And now, le rules:
-The light and dark families must NEVER actually see each other in person. They can communicate with members of the opposite family via computer or text, but they must never actually "see" each other.
-Only members of the "light" family can visit off the lot. This includes jobs, vacations, and school. The members of the "dark" family can get jobs, but they must be ones they can do from within their home.
-Members of the "dark" family must steal stuff (food mostly) from the "light" family to survive. They can go anywhere on their home lot, but they cannot leave said lot. Similarly, the "light" family must never go inside the secret places on their own home lot where the "dark" family has their base of operations.
-If members of the two families should ever see each other or talk to each other directly, the game is over. See how long you can keep the two families separate!