Parasurama (Lord Parashurama) cursed Karna to forget the invocation mantra for the Brahmastra weapon at the crucial moment when he would need it most. This curse was given after Karna lied about his birth to Parasurama, claiming to be a Brahmin when he was actually a warrior (Kshatriya). The curse meant that Karna would be unable to recall the Brahmastra when fighting Arjuna, his greatest rival. Vasista, Viswamitra, and Drona were not involved in this particular curse.