Aang

In the early ages Lord Mahadev (Shiva) was at war with a devil name ‘Andhak’. The war continued for years. After Mahadev (Shiva) killed Andhak, out of the perspiration of Lord Mahadev great spirit originated.The spirit consumed all the blood from the body of Andhak. Still his appetite was not satisfied. He then undertook penance Lord Shankar (Shiva) was satisfied by his deep penance & offered him a boon. This spirit then started eating men & animals horaciously.
All the mankind, demons and the Gods were terrified by this. They went to Lord Bramha. Lord Bramhadev asked them to lay the spirit face down. 81 Gods together laid down the spirit face down & sat on different parts of the back. When the spirit begged forgiveness from Lord Bramha. Lord Bramha gave him a boon that “After building a house or any other structure, the people who worship you as Vastudevata & give you your share of food as Balibhoj (Part of the sacrifice ) you give them all the pleasure & prosperity. However, those who start living without your prayers & offerings, you can trouble them. This & many other such myths & fables can be found in different ‘Puranas’. Probably this stories were included in ‘Puranas’ to make people conscious about the subject which would make them build there house as per Vastushastra.