Garuḍa Purāṇa 14

If one has committed innumerable sins, after his death his preta body does not transform into an airy body.  Pretas get released from Yama’s kingdom and enter the earth as ghosts.  A list of sins is enumerated is said to cause preta bodies.  They are stealing, indulgence in adultery, betrayal, etc. The pretas suffer from hunger and thirst that cannot be satiated as they cannot eat through their mouth (Elsewhere it is said that ancestors who continue to remain as subtle bodies can smell the roasting of sesame seeds.  They make all out attempts to seize the piṇḍa-s offered to other ancestors, though they cannot eat. They come back to the home where they lived and watch their kin. Since their kin cannot notice them (as they do not have any visible bodies), they get angry with their kin and even attempt to kill them (not directly but indirectly).  They also cause diseases in their family (It should be understood that attachment is only for the gross body.  When they were alive, they would have felt sad if their kin are affected by disease and when they are dead, even though they know the repercussions, still they cause the diseases).

These pretas become ghosts and cause serious afflictions to his family members.  However, the ghosts are incapable casting their spell when one worships Rudra, or regular worshippers of other gods and goddesses, who follow the path of dharma, shares food with guests and those who always speak the truth.  Pretas can cause the following miseries. Childlessness to couples, death of children at young age, loss of wealth, unexpected sufferings, tension at home, damage to crop, causing disrepute,  loss of properties due to fire or floods, incurable disease, abortions, etc. and the list is quite long. In general, ghosts can cause any damage that is not enumerated here. They are said to be responsible for accidental deaths.  

If one comes to know about the existence of ghosts, he has to perform remedial measures to pacify the ghost and release the ghost from its preta-hood.  Symptoms of presence of the ghosts are also explained.  Appearance of holy plants, mango tree with fruits, a knower of Vedas, a bull or his own image in dreams signify the presence of ghosts. In spite of his best efforts, one is not able to complete his pilgrimage it also signifies the presence of ghosts.  The ghosts become harmful if funeral rites are not performed as prescribed.

Once, rituals to get alleviate the ghost-hood are properly performed, ghost will be permanently satisfied and leave the place after blessing its kin.  Even the most dreadful ghosts go to the place of Yama and after undergoing necessary punishments, are born again. Even if there are no symptoms like dreams, etc., and if one continues to suffer due to unknown reasons, propitiating rites to Lord Viṣṇu should be performed.  If japa, homa and gifts are done properly, one will be able to get rid of his miseries caused by unknown ghosts.  

There are other methods of alleviating the ghosts.  Repeated recital of Gāyatri mantra, performance of Nārāyaṇa bali and vṛṣotsarga (release of bull discussed in earlier chapters) are the rites that are to be performed to release a ghost from its preta-hood.

Further reading: There are modern theories that confirm the presence of ghosts.  Ghosts are said to be mono dimensional in nature and generally appear exactly at a particular time in the night.  It is said that not all the ghosts are harmful.  Many ghosts have come into existence due to sudden accidents, and they are, by and large, remain un-harmful.  Ghosts can be released if one is conversant with kuṇḍalini meditation. They can, by keeping their attention on the third eye visualize the release of ghosts.  Lavender is said to be the best remedy to eliminate all evil spirits.  A drop of lavender oil mixed with water can be kept in a corner of a house.  More often than not, one’s unfounded fear is the cause for illusionary appearance of ghosts. Total surrender to God, performing charity within his means, respect to parents when they are alive, not hurting anyone either with words or with strength, leading a pious life, helping others in times of need are the qualities that can ward off evil effects of all the ghosts put together.  Unfounded fear begins to torment a person only if he had done any misdeeds.  As long as one is living a virtuous life, nothing can harm him, as God personally takes care of him and protects him all the time.