There is an interesting story in the MATSYA PURANA in which the birth of the Vastu Purusha is narrated. Vastu Purusha is , said to have been born from Lord Siva's perspiration, when the Lord was fighting the demon Andhakarusan.While fighting with the demon, Shiva was very much tired and began to sweat profusely. As a drop of Sweat fell on the ground A large dreadful being was born he was very hungry and started devouring everything and when the Demon was slain he ate him too then he prayed to lord Shiva to grant him a boon "Please permit me to eat away all the three worlds." He asked for and Lord Shiva said, "Let it be so." The devotee's joy knew no bounds. He got possession on all the three worlds and first he was ready to eat the terrestrial world. Then the celestial beings, Brahma, Vishnu and the demons (Rakshasas) also were terrified and caught hold of the devotee encircling him Forty-five celestial beings, out of whom 32 from without and 13 from within caught hold of him and threw him on the ground These celestial Beings laid force on his different limbs and Since then, he has been lying there surrounded by the celestial Beings . The gods granted him permission to eat offerings on grahpooja and also blessed him to punish thouse who do not respect him and his rules of VASTU SHASTRA and thus he was called "the God of Vastu or VASTU PURUSHA"