In layman's terms: Yes, he was possessed and, (speculation); saw a vision of the prince's death.
The tomb in which the Wight imprisoned them was noted to be the grave of the last Cardolan prince.
It was at this time that an end came of the Dunedain of Cardolan, and evil spirits out of Angmar and Rhudaur entered into the deserted mounds and dwelt there.
It is said that the mounds of Tyrn Gorthad, as the Barrowdowns were called of old, are very ancient, and that many were built in the days of the old world of the First Age by the forefathers of the Edain, before they crossed the Blue Mountains into Beleriand, of which Lindon is all that now remains. Those hills were therefore revered by the Dunedain after their return; and there many of their lords and Kings were buried. [Some say that the mound in which the Ring-bearer was imprisoned had been the grave of the last prince of Cardolan, who fell in the war of 1409.]'
The Lord of the Rings - The Return of the King, Appendix A
What Merry saw in his sleep were the visions of the late prince, who as fore-mentioned fell in the War.
'The men of Carn Dûm came on us at night, and we were worsted. Ah! the spear in my heart!'
It's likely that Merry experienced the prince's death, in a way. Merry experienced a spear in his heart, likely the cause of the prince's death.
A reference of Armor and the Witch-king is mentioned in The Fellowship of the Ring.
Then he told them that these blades were forged many long years ago by Men of Westernesse: they were foes of the Dark Lord, but they were overcome by the evil king of Carn Dum in the Land of Angmar.