Karna undoubtedly was amongst important characters of ancient and great hindu epic "Mahabharat".
As we know that Karna was biological son of "Surya" and "Kunti". He was raised by Adhirath the royal charioteer of Hastinapore and his wife Radha as Kunti being an unwed mother, had to abandon baby Karna to avoid social stigma.
I come across something amazing while studying origins of the characters of Mahabharat in Skandh Puraan , I would like to share the information with all Karna fans. I hope this new piece of information will make you aware about unheard story of Karna and Shanidev.
Surya is one among 11 Adityas and son of Sage Kashyap and Aditi is an important deity. Surya got married to Sanjya [ also called Sangya/Sanjana] daughter of Demigod Architect Vishwakarma.
Sanja bore two sons , Shani , Yama and one daughter Yamuna [ who later became river] , thereafter she couldn't bear radiance of Surya and went for penance , while leaving for penance she created her image from shadow and instructed her to look after her husband [Surya] and children in her absence. Chhaya gave birth to Tapti[ who later became river] and Savarni Manu [ The first human ]. Initially Chhaya looked after and loved Sanja's children as her own but later when she given birth to her own children , she started being partial. Shani being eldest of his siblings was very stubborn kid , he got agitated by partial attitude of Chhaya towards him and his siblings [Yama and Yamuna]. On seeing his mother holding young manu in her arms , Shani got jealous of his brother manu and tried to pull him down. Chaya scolded Shani and asked him to behave , and in a fit of anger he raised his feets to hit his mother. Chhaya got furious of the actions of Shani and immidiately cursed him to be born as mortal human , and suffer for his karma in his life in Dwapaar Yuga. She said since you tried to pull your younger brother from my arm out of jealously and just to seek my attention may you be deserted by your human mother and be alienated from your siblings. You are so proud to be son of Powerful Devtaa , may you be born to unwed woman ,face public disgrace , suffer misfortune , your human siblings unaware of your identity will hate you and never respect you , since you are jealous of your younger brother Manu.
Shani was shocked by the curse and cried , ran to his father and narrated him the whole incident sobbing complaining that "Mother doesn't loves me neither she treats us equally , so I tried to hit her in anger it was my mistake , mother got angry at this and has cursed me. Please forgive me and free me from mothers' curse"
Surya replied to Shani that a mother would never curse her child despite of 1000 crimes and that he cannot undo the curse of Mother but in his next birth as human, he will be fathered by Surya himself , he will retain some of his devta like attributes in his human incarnation but will suffer a lot , has to face condemnation and bad luck through out his life and will be liberated of Manushya Yoni by 8th avtara of Lord Vishnu.
Source : Kashi Section , Skandha Puraan
There is another account of Lord Shani's involvement in Mahabharat . In Shani Mahatmya , Lord Shani while speaking to Kali Yugan King Vikramadiyaa tells him that did not spared even Lord Krishna.
When Lord Shani entered in horoscope of Lord Krishna. People blamed Lord Krishna for stealing Syamantak Mani and killing Satrajit , when Lord Krishna realized this he worshipped Shanidev , he removed his gaze and asked Lord Krishna that he will come out of the false allegations.
Lord Krishna asked Shani that in upcoming great war between Kauravas and Pandavas which is the side that upholds dharma at this Shani replied "Pandavas". Lord Krishna requested him to favour Pandavs . Shani has given an interesting reply " I and Yama are two judges. I deliver fruit of ones' deeds through ones' life through the punishment and rewards while Yama grants fruit of one's deeds after death. Since am living curse of my mother in my human incarnation and my human incarnation is great friends with Duryodhana , I am governed by laws of karma so I can't influence the outcome of war directly or indirectly as my human incarnation has to take part in the war but as you are supreme lord I cannot deny your request and hence I am in crisis , whether to follow the duty or deny the request of lord. Since you are Ultimate Lord , please free me from this crisis situation"
Lord Krishna smiled at this and replied Shani that he will get him liberated from crisis and also ensure that his amshavatara attains salvation.
Source : Shani Mahatamya , Shreemad Bhagwat Puaranam
It is evident from above two stories that Shani was born as Karna in human form. As cursed by his mother he was immediately abandoned by Kunti , he suffered through out his life he was humiliated by Kripacharya , Dronacharya his own brothers and society. He was unfortunate to bag another 2 curses in his human life and hence Chhayas' word came true that he will suffer misfortune through his life.
Although above stories are not mentioned by Sage Vyaas in Mahabharat yet it seems quite likely that Karna is Shani's incarnation since many other characters of Mahabharat , like Bhishma, Draupadi,Ashwatthama, Pandu were in fact cursed souls. Karna had some attributes of Shani Deva like spirituality , generosity etc. Even though being in company of evil Duryodhana Karna didn't leave his devotion and spiritual qualities.
As Krishna's promise to Shani , Krishna freed Shani out of intervention by putting Karna in to crisis situation. When he was coming back from Hastinapur after negotiations failed , he disclosed identity of Karna. This is where the Karna refuses to join Pandav side by deserting Duryodhan. He cited reason about "Mitra Dharma" later when Kunti asks Karna not to kill the Pandavas , he gives a diplomatic answers that she will always be mother of five sons.
Thus Karna took beautiful decision , he did not violate "Mitra Dharm" and was able to follow his "Putra Dharm" by sacrificing his life just to keep his words which he had given to his mother.