Bereke Khan accepted Islam who defeated his cousin Hulagu Khan

1. Bereke Khan who defeated his ruthless cousin Hulaku Khan was a Muslim.
 
Ibn Kathir said:
السلطان بركه خان بن تولي بن جنكيز خان
وهو ابن عم هولاكو، وقد أسلم بركه خان هذا، وكان يحب العلماء والصالحين ومن أكبر حسناته كسره لهولاكو وتفريق جنوده
Sultan Bereke Khan bin Tawalla bin Genghis Khan was cousin of Hulaku Khan, Bereke Khan accepted Islam and he loved the scholars and the righteous. The best deed he did was to defeat Hulaku and despair his soldiers.

[al Bidaya wal Nihaya vol 13 under the people who died in 665 h]

2. When Hulaku Khan ran away with very few people and most of his army was defeated by Berke Khan and his alliance with Malik Zahir.

 Berke Khan (Grandson of Ghengis Khan) made a treaty with Malik al Zaahir against Hulaku Khan (Also a Grandson of Ghenghis) and defeated him.
 
Ibn Kathir said:
وفيها قدمت رسل بركه خان إلى الظاهر يقول له: قد علمت محبتي للإسلام، وعلمت ما فعل هولاكو بالمسلمين، فاركب أنت من ناحية حتى آتيه أنا من ناحية حتى نصطلمه أو نخرجه من البلاد وأعطيك جميع ما كان بيده من البلاد، فاستصوب الظاهر هذا الرأي وشكره وخلع على رسله وأكرمهم.
 
In this year Berke Khan sent his messenger to al Zaahir who said:
 
You know my Love for Islam and you also know what Hulaku did to Muslims. You should come on one side and I will come on the other side so that we may strike him or sent him (Hulaku) out of the country. I will give you all the cities which are captured by him. al Zaahir agreed with him and thanked his messengers and gave them respect.
 
[al Bidaya wal Nihaya 13 under the things happened in 661 h]
 
Then Ibn Kathir said:
وفيها التقى بركه خان وهولاكو ومع كل واحد جيوش كثيرة فاقتتلوا فهزم الله هولاكو هزيمة فظيعة وقتل أكثر أصحابه وغرق أكثر من بقي وهرب هو في شرذمة يسيرة ولله الحمد.
ولما نظر بركه خان كثرة القتلى قال: يعز علي أن يقتل المغول بعضهم بعضا ولكن كيف الحيلة فيمن غير سنة جنكيز خان ثم أغار بركه خان على بلاد القسطنطينية فصانعه صاحبها وأرسل الظاهر هدايا عظيمة إلى بركه خان.
And in the (same) year Bereke Khan and Hauaku Khan fought and both had great armies and they fought. Allah punished Hulaku and his most of the companions were killed and remaining drowned. Hulaku ran away with few people Alhumdulillah.
 
When Bereke Khan saw many dead bodies he said:
 
“I don’t like it when Mongols kill each other but what is heelah for the man who change the way of Genghis Khan then Bereke Khan attacked Constantinople… and al Zaahir sent many gifts to Bereke Khan.
 
[al Bidaya wal Nihaya 13 under the things happened in 661 h]
3. Death of Hulagu Khan
 
Ibn Kathir said:
وفيها: جاءت الأخبار بأن سلطان التتار هولاكو هلك إلى لعنة الله وغضبه في سابع ربيع الآخر بمرض الصرع بمدينة مراغة،
In this year (i.e. 663 h) the news reached that the Sultan of Tatar Hulaku died, Allah’s curse and anger be upon him on 7th Rabi al Akhir in the disease of Epilepsy.
 
[al Bidaya wal Nihaya 13 under the things happened in 663 h]
 Ibn Kathir mentioned that he died in 664 h and also said some say he died in 663 h.
4. Help of Breke Khan for Muslims when A Christian reverted to Islam and Christians from the help of Mongols tortured him and Martyred him.

al-Jūzjānī, who claims to have heard the story, while in Delhi, from the lips of a certain Sayyid Ashraf al-Dīn who had come there from Samarqand.

“The eminent Sayyid thus related, that one of the Christians of Samarqand attained unto the felicity of Islam, and the Musalmans of Samarqand, who are staunch in their faith, paid him great honour and reverence, and conferred great benefits upon him. Unexpectedly, one of the haughty Mongol infidels of China, who possessed power and influence, and the inclinations of which accursed one were towards the Christian faith, arrived at Samarqand. The Christians of that city repaired to that Mongol, and complained saying: ‘ The Musalmans are enjoining our children to turn away from the Christian faith and from serving Jesus—on whom be peace—and calling upon them to follow the religion of Muṣ ṭ afặ 1—on whom be peace— and, in case that gate becomes unclosed, the whole of our dependents will turn away from the Christian faith. By thy power and authority devise a settlement of our case. The Mongol commanded that the youth, who had turned Musalman, should be produced, and they tried with blandishment and kindness, and money and wealth to induce the newly-converted Musalman to recant, but he refused to recant, and put not off from his heart and spirit that garment of freshness—the Muslim faith. The Mongol ruler then turned over a leaf in his temper, and began to speak of severe punishment; and every punishment, which it was in his power to inflict, or his severity to devise, he inflicted upon the youth, who, from his great zeal for the faith of Islam, did not recant, and did not in any way cast away from his hand the sweet draught of religion through the blow of infidel perverseness s. As the youth continued firm in the true faith, and paid no heed to the promises and threats of that depraved company, the accursed Mongol commanded that they should bring the youth to public punishment; and he departed from the world in the felicity of religion—may God reward and requite him !” [Jūzjānī, pp. 448-50. Raverty, pp. 1288-90]

Muslims asked Bereke Khan to help them.

The quote continues

“and the Musalman community in Samarqand were overcome with despondency and consternation in consequence. A petition was got up, and was attested with the testimony of the chief men and credible persons of the Musalman religion dwelling at Samarqand, and we proceeded with that petition to the camp of Baraka Khān, and presented to him an account of the proceedings and disposition of the Christians of that city. Zeal for the Muslim religion was manifested in the mind of that monarch of exemplary faith, and the defence of the truth became predominant in his disposition. After some days, he showed honour to this Sayyid, appointed a body of Turks and confidential persons among the chief Musalmans, and commanded that they should slaughter the Christian company who had committed that dire oppression, and despatch them to hell. When that mandate had been obtained, it was preserved until that wretched sect had assembled in the church, then they seized them all together, and despatched the whole of them to hell, and reduced the church again to bricks.”

[Jūzjānī, pp. 448-50. Raverty, pp. 1288-90, Taken from THE PREACHING OF ISLAM by Thomas Walker Arnold page 171]
5. After 29 years most of the Tatar (Mongols) accepted Islam.
 
Ibn Kathir said:
 
وفيها: ملك التتار قازان بن أرغون بن أبغا بن تولى بن جنكيز خان فأسلم وأظهر الإسلام على يد الأمير توزون رحمه الله، ودخلت التتار أو أكثرهم في الإسلام، ونثر الذهب والفضة واللؤلؤ على رؤوس الناس يوم إسلامه، وتسمى بمحمود.
In this year (i.e. 694 h) The king of Tatar Qazaan bin Arghoon bin Abgha bin Tawalla bin Genghis Khan accepted Islam. He accepted Islam on the hands on Ameer Tozoon Rahimaullah, Tatar or most of them accepted Islam. He (Qazaan) spent Gold, Silver and pearl on people on the day of his acceptance (i.e. due to happiness) and named himself Mahmood.
 
[al Bidaya wal Nihaya vol 13 under the year 694 h]