Is it obligatory to fast and pray when black blood is discharged 18 days after an irregular menstrual period?

If the blood stops for thirteen days or more, the returning blood is considered menses. However, if it stops for less than thirteen days, it is considered istihadah (prolonged non-menstrual bleeding). In this case, the woman performs ablution after the time for prayer begins and prays whatever obligatory and supererogatory prayers she wishes until the time for prayer expires. She also fasts and has all the rulings of ritually pure women.