Is the torment of the grave a matter of the Unseen or the Seen, and what is the wisdom behind it being a matter of the Unseen?

The torment of the grave is among the matters of the unseen (Ghaib), and had it not been for revelation, we would have known nothing about it. Allah may disclose it to whomever He wills. The wisdom behind it being unseen is: Allah's mercy upon people by not letting them witness the torment of their loved ones, Allah's concealment of the deceased by not exposing them, facilitating the burial of the dead, and that belief in it carries greater merit because it is unseen, and because seeing the torment might make people believe out of necessity, not out of choice. Thus, the tidings of Allah are more truthful than observation.