When number of stars concentrate at one particular position in a horoscope then it is termed as conjunction of planets (yog). In astrology "yog" has paramount importance.
Before we study kalsarpa, it is important to take into account Rahu (North Lunar Node) and Ketu (South Lunar Node). Rahu is considered as a head of a serpent where as Ketu is said to be the tail of the serpent. Because of these two planets, kalsarpa yog is formed. Lord Rahu, son of Sinihsika and Hiranya Kasyap submerged himself into the sea to imbibe nectar obtained from the Samudramathan. when Rahu was about to drink Lord Surya and Chandra (Sun and Moon) hinted to Mohini. Lord Vishnu disguised as Mohini slashed his head off. After this the north part is called as Rahu while the south part is called as Ketu. Here after they were made part of the Navgraha.
Kalsarpa yog is formed when rahu is tainted. When Rahu is in auspicious position then it brings good fortune and is heroic. It's supremacy lies in charms and spells, black magic. If Rahu is tainted then it could lead to memory loss, ill fame, demoniac possession, and danger to descendent. Also can lead to delay and failure in different work areas. To make the task easier and achievable Vedic Kalsarpa Yog Shanti Vidhan is to be performed.