The Tree of Zaqqum is a torment inside of the Hellfire that Allah (SWT) has prepared for those who disbelieve.

The Holy Quran says:
17.60 And [remember, O Muhammad], when We told you, “Indeed, your Lord has encompassed the people.” And We did not make the sight which We showed you except as a trial for the people, as was the accursed tree [mentioned] in the Qur’an. And We threaten them, but it increases them not except in great transgression.

37.61 For the like of this let the workers [on earth] work.
37.62 Is Paradise a better accommodation or the tree of zaqqum?
37.63 Indeed, We have made it a torment for the wrongdoers.
37.64 Indeed, it is a tree issuing from the bottom of the Hellfire,
37.65 Its emerging fruit as if it was heads of the devils.
37.66 And indeed, they will eat from it and fill with it their bellies.
37.67 Then indeed, they will have after it a mixture of scalding water.
37.68 Then indeed, their return will be to the Hellfire.

44.43 Indeed, the tree of Zaqqum
44.44 Is food for the sinful.
44.45 Like murky oil, it boils within bellies
44.46 Like the boiling of scalding water.
44.47 [It will be commanded], “Seize him and drag him into the midst of the Hellfire,
44.48 Then pour over his head from the torment of scalding water.”
44.49 [It will be said], “Taste! Indeed, you are the honored, the noble!
44.50 Indeed, this is what you used to dispute.”

56.51 Then indeed you, O those astray [who are] deniers,
56.52 Will be eating from trees of zaqqum
56.53 And filling with it your bellies
56.54 And drinking on top of it from scalding water
56.55 And will drink as the drinking of thirsty camels.
56.56 That is their accommodation on the Day of Recompense.

In the Quran, the Tree of Zaqqum is mentioned in these four separate surahs (chapters), and I can never even imagine describing it as it’s shown in the Quran.

A Hadith says:
Narrated Ibn Abbas: Regarding: ‘And We granted the vision (Ascension to the Heaven “Miraj”) which We showed you (O Muhammad as an actual eye witness) but as a trial for mankind.’ (17.60) It was an actual eye-witness which was shown to Allah’s Apostle during the night he was taken on a journey (through the heavens). And the cursed tree is the tree of Az-zaqqum (a bitter pungent tree that grows at the bottom of Hell). (Book #60, Hadith #240) [Bukhari]

So here we have so many warners of these punishments, and we still disbelieve, so are we not defiantly disobedient? May Allah (SWT) save us all from His Wrath and may He admit us into His Mercy. Ameen.