I am a Christian girl with two children, going to convert to Islam. I am getting divorced due to my husband's affair. My husband has had a lot of affairs before and after marriage with me. Now I am going to marry my old friend; he is a Pathan. He told me that after talaq with my husband, I have to sleep with someone as a tradition. This is a punishment for my existing husband. I asked him how it will be a punishment for him; it's a punishment for me. I don't want to sleep with anyone. My friend says that, in Islam, it is the tradition in Karachi, Pathan.
...I have to sleep with someone as a tradition.
This would be zina (fornication), a major sin in Islam.
Zina is an Islamic law concerning unlawful sexual relations between Muslims who are not married to one another through a nikah (Islamic marriage). ... [It] is not allowed by Sharia (Islamic law) and classed as a hudud crime (class of Islamic punishments that are fixed for certain crimes that are considered to be "claims of God"). (Wikipedia)
It is forbidden in the Qur'an:
And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty. -- Qur'an 25:68-70
Are you sure you didn't mishear? It is not appropriate for a Muslim to promote sin:
Hudhaifah (May Allah bepleased with him) reported: The Prophet (ﷺ) said, "By Him in Whose Hand my life is, you either enjoin good and forbid evil, or Allah will certainly soon send His punishment to you. Then you will make supplication and it will not be accepted". (sunnah.com)
Also, I'm a recent convert, and I found this advice useful:
For a new Muslim, this author would advise to wait at least a year, preferably more, before thinking about getting married. -- NewMuslims.com
I don't plan to find a husband until I'm well established in my faith.
There's a lot to learn about Islam, and, in particular, I encourage you to learn about your rights and responsibilities as a wife before getting married. This affects you and your children, e.g., what if he decides you're moving to another country?
All Praise to Allah Subhanahu wa Taala and blessings of Him be on Prophet Muhammad peace be upon him His family and companions,
There is no such Tradition in Karachi city or Pakistan neither in Pathan Community you need to do clarification in this regard and need to learn Islam by some Scholar or visit your nearby Mosque.
May be you would have misunderstood probably he would be trying to inform you about Nikah Halala as one of the option if in future you want to remarry your husband or He might got confused or has wrong information as it is not intented for the punishment in such scenario.
It was narrated that Ibn 'Umar said: "The Prophet was asked about a man who divorced his wife three times, then another man married her and he closed the door and drew the curtain, then divorced her before consummating the marriage with her. He said: "She is not permissible for the first one (to remarry her) until the second one has had intercourse with her."(Nasai)
'A'Asha (Allah be pleased with her) reported: A person divorced his wife by three pronouncements; then another person married her and he also divorced her without having sexual intercourse with her. Then the first husband of her intended to remarry her. It was about such a case that Allah's Messenger (ﷺ) was asked, whereupon he said: No, until the second one has tasted her sweetness as the first one had tasted.(Muslim)
Allah Swt and His Messenger knows best.