Merry was dreaming of Pelennor again.
More specifically, he was dreaming that he was back in Gondor, after that battle, wandering the streets of Minas Tirith. Lost in the dark. And, in his dream, he heard Boromir and Theoden calling to him from somewhere a long, long way away, wanting him to join them.
His dreams never reached the point where Aragorn had driven away the poison. In them, there was nothing but the dark and the voices of the dead.
Merry woke with a cry and found himself cradled safe and warm in a pair of strong arms, with Pippin stroking his hair.