Abu Amir al-Rahib, "the Monk," Abd Amr ibn Sayfi, was a Christian leader of the Aws in Yathrib whose expected leadership slipped away when the Prophet ﷺ arrived, and who refused Islam; his son Hanzalah RA became a famous martyr. He left for Makkah with some followers, and at Uhud called on the Aws to abandon the Prophet ﷺ, only to be cursed as Abu Amir "the wicked." He later went to the Romans and urged Ibn Ubayy to build the base that became Masjid al-Dirar, destroyed after Tabuk. He died a stranger in the land of the Romans, as the Prophet ﷺ had prayed.
All 2 events
وَالَّذِينَ اتَّخَذُوا مَسْجِدًا ضِرَارًا وَكُفْرًا وَتَفْرِيقًا بَيْنَ الْمُؤْمِنِينَ وَإِرْصَادًا لِّمَنْ حَارَبَ اللَّهَ وَرَسُولَهُ مِن قَبْلُ ۚ وَلَيَحْلِفُنَّ إِنْ أَرَدْنَا إِلَّا الْحُسْنَىٰ ۖ وَاللَّهُ يَشْهَدُ إِنَّهُمْ لَكَاذِبُونَ
“And [there are] those [hypocrites] who took for themselves a mosque for causing harm and disbelief and division among the believers and as a station for whoever had warred against Allah and His Messenger before. And they will surely swear, "We intended only the best." And Allah testifies that indeed they are liars.”
The verse's "station for whoever had warred against Allah and His Messenger before" refers to Abu Amir al-Rahib.