Starting with an unrelated ghost and child they need to find their way in life or unlife.
The end goal is for the child to survive through their teenage years and bring the ghost back to life.

Set up:
Make sure your [young adult-elder] sim is a ghost. It doesn't matter how but the challenge doesn't start until they are a ghost.
They should be on any empty lot ( If you want plants for ambiance if you remove them you should remove twice the amount of simoleons you get for them.)

The rules are:
1.) There can be no other sims except for the child and the ghost.
2.) The child's trait is insane (The rest don't matter.)
3.) The ghost cannot interact with objects other than to fulfill needs (which I don't think they should have but that's another thing.)
4.) The ghost cannot interact with other sims or get a job. No one but the child can see or hear them. (They'd need to develop an attachment beforehand.)
5.) If it hasn't been established then the child should be the moneymaker with the ghost as a dependent.
6.) Like with the runaway ten challenge, no building until you have the simoleons to do so houses/shacks don't tend to go up wall piece by wall piece.
7.) No build mode until the child is a teen though. No counters and no expensive appliances or furniture. Who would sell all that stuff to a kid? At least a teen can attempt to pass as an adult.
8.) Just because time limits are always fun, the child can be a vampire.
9.)If the child is not a vampire then age restoration of the child is not allowed.
10.) No attempts to bring the ghost back to life until the child and the ghost are best friends (All in game methods are cool though.)
11.) No using other peoples stuff. If a filthily child kept coming to one's house to eat, use the bathroom, and sleep one would probably try to help them and that kind of help wouldn't get the ghost brought back to life.
12.) No using community lots. See eleven.
13.) Sleeping on benches is fine though, as is eating left out food.
14.) Any food that the ghost eats has to be gotten or found by the child. i.e. If you want the ghost to eat from a cooler have the child pull the food out and put it down. The ghost cannot eat without the child on the lot.
15.) The ghost cannot cook or clean.
16.) The ghost cannot call people on the phone (or use it to be honest, but try to stop a sim from using a phone with free will on) the sound transfers as spectral noise and freaks people out, it is a good way to get exorcised.
17.) It might be better to turn off freewill until the child is on the lot.
18.) Ghosts don't age, if you find that your ghost is aging, find a way to raise someones aspiration points enough to buy a potion of youth before they age up.
19.) Once the child ages up to a teen pay your bills BEFORE buying or building anything else.
21.) NO CHEATS THAT MAKE THE CHALLENGE EASIER!

If your child sim dies or is taken before bringing back your ghost it's a loss.
Have fun!