Due to this curse they were born as Hiranyaksa and Hiranyakasipu in their next birth. Hiranyaksa was very awe-inspiring. It was because of him that the Prithvi went hiding in Patala loka from where Varaha avatar of Vishnu had to bring her back. Varaha eventually killed Hiranyaksa and rescued people from his terror.
Same goes with Hiranyakasipu. He was blessed by Brahma that nobody in sky or on earth, during day or night, no god of demon can ever kill him. His son Prahlada was a devotee of Vishnu and Hiranyakasipu was not amused by it. Eventually Vishnu came forth to rescue Prahlada and others from him as Narsingh avtara and killed him. Narsingh was neither devta nor raksas, he killed Hiranyakasipu during evening which was neither day nor night and he held Narsingha in such a way that he was neither in sky nor on earth then. Thus Brahma’s boon was nullified.
In Treta Yuga both of these brothers were again born as Ravana and Kumbhakarna, when they were killed by Rama, another incarnation of Vishnu. Finally in Dwapara Yuga, they were born as Shishupal and Dantvaktra, only to be killed by Krishna, the poorna avtara.