Diti replied: O my husband, I have now lost my sons. If you want to give me a benediction, I ask you for an immortal son who can kill Indra. I pray for this because Indra, with the help of Visnu, has killed my two sons Hiranyaksa and Hiranyakasipu

Expressions researched:
"Diti replied: O my husband, O great soul, I have now lost my sons. If you want to give me a benediction, I ask you for an immortal son who can kill Indra. I pray for this because Indra, with the help of Viṣṇu, has killed my two sons Hiraṇyākṣa and Hiraṇyakaśipu"

Srimad-Bhagavatam

SB Canto 6

Diti replied: O my husband, O great soul, I have now lost my sons. If you want to give me a benediction, I ask you for an immortal son who can kill Indra. I pray for this because Indra, with the help of Viṣṇu, has killed my two sons Hiraṇyākṣa and Hiraṇyakaśipu.

SB 6.18.37, Translation and Purport

Diti replied: O my husband, O great soul, I have now lost my sons. If you want to give me a benediction, I ask you for an immortal son who can kill Indra. I pray for this because Indra, with the help of Viṣṇu, has killed my two sons Hiraṇyākṣa and Hiraṇyakaśipu.

The word indra-haṇam means "one who can kill Indra," but it also means "one who follows Indra." The word amṛtyum refers to the demigods, who do not die like ordinary human beings because they have extremely long durations of life. For example, the duration of Lord Brahmā's life is stated in Bhagavad-gītā: sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ (BG 8.17). Even the duration of one day, or twelve hours, of Brahmā is 4,300,000 years multiplied by one thousand. Thus the duration of his life is inconceivable for an ordinary human being. The demigods are therefore sometimes called amara, which means "one who has no death." In this material world, however, everyone has to die. Therefore the word amṛtyum indicates that Diti wanted a son who would be equal in status to the demigods.