The sAnskrit works, mahAbhArata tAtparya nirNaya (13th century) (2) and nArAyaNIyam (16th century) (3) mention karNa as the incarnation of sahasrakavacha (one with thousand armours).
In mahAbhArata tAtparya nirNaya , madhvAchArya states as follows:
purA sa vAlimAraNaprabhUtadoShakAraNAt |
sahasravarmanAminA.asureNa veShTito.ajani || [ 11-150 ]
Because of the stigma attached to him in his becoming instrumental to eliminate his own brother vAli, sugrIva took birth as karma, together with the life force of a demon named sahasravarman.
Thus according to madhvAchArya, the demon sahasravarman contributed to the life force of karNa. karma appears to be a combined incarnation of sugrIva and sahasravarman.
nArAyaNIyam is a condensation (1036 slokas) of bhAgavata. In nArAyaNIyam,
melputtUr nArAyaNa bhaTTatiri states as follows:
daityaM sahasrakacahaM kavachaiH parItam
sAhasravatsaratapaH samarAbhilavyaiH |
paryAyanirmitatapaH samarau bhavantau
shiShTaikaka~NkaTamamuM nyahatAM salIlam || [ 16-3 ]
nara and nArAyaNa battled, by turns, with the daitya (asura ) sahasrakavacha who was having one thousand armours, each of which could be cut off with an year of asceticism and an year of battle. In this way 999 armours were destroyed and one remained.
The commentators of nArAyaNIyam note (page 114) as follows:
This sahasrakavacha is born as karNa. The slaying of sahasrakavacha is not described in bhAgavata. The poet (melputtUr nArAyaNa bhaTTatiri) has taken this episode from mahAbhArata.
This mahAbhArata could be the southern recension. This sloka in nArAyaNIyam occurs in the context of the incarnation of nara and nArAyaNa (bhAgavata, 4th canto,chapter 1).
karNa is desribed as an incarnation of sahasrakavacha in the malayALaM poem. kR^iShNArjunavijayaM (18th century) (4) which deals with gayopAkhyAnam. This is a popular work in malayALaM by ambayATTu paNikkar, a disciple of kunchan nambiyAr (AD 1700 - 1770).
There was a daitya (asura), sahasrakavacha (one with a thousand armours), destroyer of devas. Due to a boon given by brahma, no one was ready to fight him. No harm will come to him without the loss of the thousand armours. After doing penance for twelve years and then battling for twelve years, one armour can be destroyed. That was the boon. To kill the asura, viShNu with thousand names incarnated as nArAyaNa and nara on earth. nara did penance and nArAyaNa battled. After twelve years one armour was destroyed. Then nara battled and nArAyaNa did the penance. In this way the battle was fought for several years and at last, 999 armours were destroyed and only one remained. Then the deluge (praLaya) occurred. nara and narAyaNa attained viShNupada and the asura took shelter with sun. karNa, surya's son is the incarnation of the asura. Arjuna and kR^iShNa are nara and narAyaNa. Both are required to kill the son of surya. The stay in forest is the penance for twelve as required.
Thus in this version, the boon is given by brahma and twelve years of penance and battle are required to destroy one armour. Twelve years of vanavAsa by arjuna are equated to penance.
Besides the Tamil tradition of karNa as the incarnation of tanAsura, we have two sAnskrit works and a Malayalam poem depicting the same tradition with minor variations. One may also look at retellings of gayopAkhyanam in Telugu, Kannada etc. The connection of karNa with sahasrakavacha appears also in yakShagAna (5).
(1) Hanne M.De Bruin, Life outside the Text: On the background and Interpretation of the Mahabharata Characters in Contemporary kaTaikUttu Performances, Text and Variations of the Mahabharata., Edited by Kalyan kumar Chakravarty, pages 235-246 National Mission for Manuscripts, New Delhi, 2009.
(2) srImahAbhAratatAtparyanirNaya is available on the internet at http://www.dvaita.org/sources/mbtn/index.shtml
For detailed information and English translation, visit http://www.mahabharatatatparyanirnaya.com/home
(3) srimannArAyaNIyaM, nArAyaNa bhaTTapAdar, vanamAlAvyAkhyAnam, guruvayur devaswam, guruvayur, Fifth Edition, 2006.
(4) ezhupattanchu tullalkathakal, edited by Dr. V.S.Sharma, page 834, D.C.Books, Kottayam, 1976.
paNToru daityan devakulAntaka-
nuNTAyvannu shasram kavachan
naShTaM vannu bhavikkayumillA
pantIrANDu tapassum chaitoru
pantIrANTatha yuddhaM chaitAl
oru kavachaM punaraRuti varuttAM
perukina varabalamorttAluM nI
naranuM chennu tapassu tuTa~N~Ni
narAyaNanum porutu tuTa~N~Ni
pantIrANDu kazhi~n~noru sheSham
antaM vannitu kavachavumonnu
naranum pinnepporu tuTa~N~Ni
nArAyaNanum tapasA mevi
evaM kazhi~n~nu palakAlam samaraM chaitu
devAri kavacha~N~NaLAviLaM nAshaM chaitu
toLLAyiravuM pinne toNNUTTonpatuM poyi
kaLLamozhi~n~nu pinneyonne kavachamullU
brahmamAm viShNupadaM nirmalanmAruM chernnu
daityan poyAdityanoTaikyavuM prApichchitu
bhoShkalla karNanAya mArttANDaputranatre
neratre phalgunanuM narakAri kR^iShNan tAnum
iruvaruM veNaM ravijanekkonnITuvAn
vanavAsaM panTIrANTu tapassAy vannukUTi