Adhab-Algal: What is it?

The Mohammedan purgatory, where the wicked are tormented by the dark angels Munkir and Nekir.

That's raises another question: who is Munkir and Nekir??

When a person is dead, then after his burial and if not buried then in whatever condition and manner the dead body has been disposed off, two angels come to him. One of them is called Munkir and the other Nakeer or Nekir. They enquire from the dead as to who is his Creator (Allah), what was his religion and who is Prophet Muhammad Sallallaahu alayhi wasallam.

If the deceased has been a faithful believer, his replies are correct and proper then he gets peace and amenities of all kinds. They open a window in his grave towards the Heaven and the cool, sweet and fragrant air of the Heaven enters his grave and the dead sleeps peacefully.
But if the dead was not a faithful believer or had no Faith (Imaan), in reply to all the questions of the angels he says that he knows nothing. Then such a person is subjected to great hardships and punishment till the Day of Judgement.