In his previous birth Karna received a boon that he can survive death a 1000 times.
And to kill him once one has to pray to god for 1000 years.
Then how did he die?
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he was dambhodbhav demon in last birth...as u know nar & narayan who were arjun & krishna of next birth defeated him...dambhodbhav got boon of 1000 sheilds & whoever break sheild will die...nar used to fight 1000 yrs & narayan used to meditate then when nar break the sheild & die the narayan by mediation's punya used to bring nar back to life & then nar meditate & narayan fight & so on they broke 999 sheilds but then with 1 sheild left the demon ran to abode of sun & then took birth of karna...& it is also said that after karna birth karna took a birth of devotee of lord shiva who was siru thonda nayanar...as per south indian beliefs...:)