If I understand your question the most prohibited thing in Islam which the prophet muhammad tells us about is to associate partners beside Allah
And then after that the major sins
You can know them from this ayah
{۞ قُلْ تَعَالَوْا أَتْلُ مَا حَرَّمَ رَبُّكُمْ عَلَيْكُمْ ۖ أَلَّا تُشْرِكُوا بِهِ شَيْئًا ۖ وَبِالْوَالِدَيْنِ إِحْسَانًا ۖ وَلَا تَقْتُلُوا أَوْلَادَكُم مِّنْ إِمْلَاقٍ ۖ نَّحْنُ نَرْزُقُكُمْ وَإِيَّاهُمْ ۖ وَلَا تَقْرَبُوا الْفَوَاحِشَ مَا ظَهَرَ مِنْهَا وَمَا بَطَنَ ۖ وَلَا تَقْتُلُوا النَّفْسَ الَّتِي حَرَّمَ اللَّهُ إِلَّا بِالْحَقِّ ۚ ذَٰلِكُمْ وَصَّاكُم بِهِ لَعَلَّكُمْ تَعْقِلُونَ (151) وَلَا تَقْرَبُوا مَالَ الْيَتِيمِ إِلَّا بِالَّتِي هِيَ أَحْسَنُ حَتَّىٰ يَبْلُغَ أَشُدَّهُ ۖ وَأَوْفُوا الْكَيْلَ وَالْمِيزَانَ بِالْقِسْطِ ۖ لَا نُكَلِّفُ نَفْسًا إِلَّا وُسْعَهَا ۖ وَإِذَا قُلْتُمْ فَاعْدِلُوا وَلَوْ كَانَ ذَا قُرْبَىٰ ۖ وَبِعَهْدِ اللَّهِ أَوْفُوا ۚ ذَٰلِكُمْ وَصَّاكُم بِهِ لَعَلَّكُمْ تَذَكَّرُونَ (152) وَأَنَّ هَٰذَا صِرَاطِي مُسْتَقِيمًا فَاتَّبِعُوهُ ۖ وَلَا تَتَّبِعُوا السُّبُلَ فَتَفَرَّقَ بِكُمْ عَن سَبِيلِهِ ۚ ذَٰلِكُمْ وَصَّاكُم بِهِ لَعَلَّكُمْ تَتَّقُونَ (153) ثُمَّ آتَيْنَا مُوسَى الْكِتَابَ تَمَامًا عَلَى الَّذِي أَحْسَنَ وَتَفْصِيلًا لِّكُلِّ شَيْءٍ وَهُدًى وَرَحْمَةً لَّعَلَّهُم بِلِقَاءِ رَبِّهِمْ يُؤْمِنُونَ (154) وَهَٰذَا كِتَابٌ أَنزَلْنَاهُ مُبَارَكٌ فَاتَّبِعُوهُ وَاتَّقُوا لَعَلَّكُمْ تُرْحَمُونَ (155)** [الأنعام : 151-155]
( 151 ) Say, "Come, I will recite what your Lord has prohibited to you. [He commands] that you not associate anything with Him, and to parents, good treatment, and do not kill your children out of poverty; We will provide for you and them. And do not approach immoralities - what is apparent of them and what is concealed. And do not kill the soul which Allah has forbidden [to be killed] except by [legal] right. This has He instructed you that you may use reason."
( 152 ) And do not approach the orphan's property except in a way that is best until he reaches maturity. And give full measure and weight in justice. We do not charge any soul except [with that within] its capacity. And when you testify, be just, even if [it concerns] a near relative. And the covenant of Allah fulfill. This has He instructed you that you may remember.
( 153 ) And, [moreover], this is My path, which is straight, so follow it; and do not follow [other] ways, for you will be separated from His way. This has He instructed you that you may become righteous.
( 154 ) Then We gave Moses the Scripture, making complete [Our favor] upon the one who did good and as a detailed explanation of all things and as guidance and mercy that perhaps in [the matter of] the meeting with their Lord they would believe.
( 155 ) And this [Qur'an] is a Book We have revealed [which is] blessed, so follow it and fear Allah that you may receive mercy.
6:151/155
Those are the major sins in Islam if I understood the question .. my English is not that good
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