Forgot 'Al HaNissim' on Purim Night
Question
Answer
Hello and happy Purim
According to Halacha, whether one forgot during the day or night, in prayer or in Birkat Hamazon, he does not repeat it.
If he remembers before finishing the blessing, he returns and says it.
In Birkat Hamazon, if he forgot and remembered while in the fourth blessing, he says before 'HaRachaman', 'HaRachaman, may He perform for us miracles and wonders as He did for our forefathers in those days at this time. In the days of Mordechai, etc.'.
Source
Shulchan Aruch Orach Chaim, Siman 695
Mishna Berura, Seif Katan 15
Biur Halacha, Siman 114, Seif 6
However, regarding 'Al HaNissim' on Chanukah and Purim, if one forgot and remembered before finishing the blessing of 'HaTov Shimcha', it implies from both that he should return and say afterwards 'Ve'al Kulam Yitbarech', etc., in order.