After you saved Regulus from the hand of the Inferi in the cave where one of dark lord's soul pieces was hidden in a locket, you took Regulus home with you barely alive and gave him water to drink, after which he quickly fell asleep, exhausted after the events.

It is now middle of the night and he's having nightmares, sweating profusely. You decide maybe you should take off his clothes, which you did not dare to do earlier. As you gingerly try to undo the buttons of his shirt, he mumbles and moans in his sleep:

"Destroy the locket..." and "Noooo!" after which it seems like he's suffocating, as if he was drowning in the lake's black waters all over again.