What is the expiation for someone who verbally abused people without their knowledge, then repented, but is unable to ask for their forgiveness? Can Allah reconcile between them on the Day of Resurrection and forgive him while concealing his sin from them, especially if they were his parents?

Allah has forbidden backbiting and likened the one who engages in it to someone who eats the flesh of his dead brother. The Prophet, peace be upon him, defined backbiting as "your mentioning your brother with that which he dislikes." If what you say about him is true, then you have backbitten him; but if it is not true, then you have slandered him. Backbiting one's parents is even more reprehensible.

Repentance from backbiting is valid if the conditions of repentance are met: cessation, regret, and determination not to return to it. Since backbiting relates to the rights of human beings, one must seek forgiveness from the one who was backbitten. It is best to ask for forgiveness and pardon from one's parents. However, if doing so would lead to harm and animosity, then it is sufficient to seek forgiveness for them, strive to be righteous and kind to them, and increase in good deeds. One should also mention their parents positively, supplicate for them in their absence, and seek forgiveness for them. Sincere repentance erases what came before it, and "the one who repents from sin is like one who has no sin."