The greatest of these sins is shirk (associating others with Allah), which leads to doom with no hope of redemption, and if a person dies in this state he will abide in Hell for all eternity. Allah, may He be exalted, says (interpretation of the meaning):

“Verily, whosoever sets up partners in worship with Allah, then Allah has forbidden Paradise for him, and the Fire will be his abode. And for the Zalimun (polytheists and wrongdoers) there are no helpers.” [al-Maidah 5:72]

“And indeed it has been revealed to you (O Muhammad (blessings and peace of Allah be upon him)), as it was to those (Allah’s messengers) before you: "If you join others in worship with Allah, (then) surely (all) your deeds will be in vain, and you will certainly be among the losers.” [az-Zumar 39:65]

“It is not for the Mushrikun (polytheists, idolaters, pagans, disbelievers in the Oneness of Allah), to maintain the Mosques of Allah (i.e. to pray and worship Allah therein, to look after their cleanliness and their building, etc.), while they witness against their own selves of disbelief. The works of such are in vain and in Fire shall they abide.” [at-Tawbah 9:17]

2. Witchcraft
Witchcraft involves shirk because it is worshipping the jinn and is seeking the help of the jinn to misguide people.

The practitioner of witchcraft is the one who deals with that which causes harm to people by means of the jinn and worshipping them instead of Allah. Sometimes he does things that harm them of words, deeds and blowing on knots; sometimes he does that by causing illusions to such an extent that a thing may be seen as something other than what it really is, as Allah, may He be glorified, said concerning the sorcerers of Pharaoh (interpretation of the meaning):

“…their ropes and their sticks, by their magic, appeared to him as though they moved fast.” [Ta-Ha 20:66]

“So when they threw, they bewitched the eyes of the people, and struck terror into them, and they displayed a great magic.” [al-A'raf 7:116]

Sometimes the practitioner of witchcraft does things to harm people by means of the jinn and worshipping them instead of Allah, by means of words, deeds and blowing on knots, as Allah, may He be exalted, says (interpretation of the meaning): “And from the evil of the witches when they blow in the knots.” [al-Falaq 113:4]

And sometimes they do that by creating illusions, so that they cause things to be seen as something other than they really are. So a rope or a stick may be seen as a snake, or a rock may be seen as an egg, or a person may be seen as something other than he really is, and so on. Such a person is one of the disbelievers and if it is proven before the judge, it is obligatory for the ruler of the Muslims to execute the practitioner of witchcraft.