Siddhars are those who lived and maintained their bodies as they desire best. They were mostly of a Tamil Saiva sect. Which maintained Siva for its God and rejected everything else in the Saiva system which was inconsistent with the pure Theism. They had investigated that the body, though transient was the one and the only instrument for attaining success in the spiritual development and growth and so worked out to attain the eight supernatural powers, like Anima, Mahima, Laghima, etc., essential for their goal, as mentioned in Silappathigaram. They further realized that if the body could only be made strong and perfect they could get rid of birth and death and live for ages together. They are the greatest men holding tremendous powers in themselves by way of yoga practice and rejuvenation. They are capable of enjoying spiritual success in their mortal bodies.
The further attained Spiritual awakening by rousing, with their suppressed breathing, the Kundalini lying dormant at the base of the spinal column in the region of the sacral plexus; and with the help of remedies of high potencies ad virtues consisting of all kinds of drugs, minerals, metallic preparation and poisons they sought of Elixir of life, thus conquering all infirmities against grey hairs, wrinkles of the skin, old age, and death. They could fly through the aerial regions with the aid of animated mercurial pills and even visited all the planets and stars and enjoyed the pleasures and wonders found therein.
So they are men holding tremendous and extraordinary powers, superhuman in nature, and can even, in this life, change their bodies into god bodies. There are several such cases mentioned by siddhars in their books. They can change the material of their bodies and rearrange the molecules in such a fashion that they may have no more sickness or death. They can send mental electricity without the aid of nerve channels, thus holding communion by remaining in their own places, with anybody and anywhere in the Universe. Their principle is that ideality and spiritual knowledge can only be attained through Yoga and to reach this, bodily existence is essential and so, that wanted to gain success before they die. They realized that if the body came to an end, it would take a longer time to reach the goal by being born again and again, and in this way, much time will be lost.