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Lanat is not just a prayer for removal of mercy, but it is a curse which descends upon the one on whom lanat is said.
The below text taken from Ziyarat of Ashoora as narrated in Kaamil al-Ziyarat:
اللّهُمّ إنَّ هذا يَومٌ تَنزَّلَتْ فيهِ اللَّعْنَةُ عَلى آلِ زيادٍ وَآلِ اُمَيَّةَ وَابْنِ آكِلَةِ الأكْبادِ
"O Allah (swt), this is the day in which you send down curses upon the descendants of Ziyaad and descendants of Marwan and descendants of Umayya the son of the eaters of livers."
[Source: Kamil al-Ziyarat, Chapter. 71, Hadith. 7]
Similar wordings are found in another "Ziyarat al Ashoora al-Ghair al-Mashhoor" found in Mafateeh al-Jinaan narrated from "al-Mizaar al-Qadeem".