Regulus knew he was going to die. He knew James and Sirius didn't care and wouldn't come and rescue him. He knew that they probably looked at the letters he sent and didn't even open them. He knew he was going to die. And there was no way around it. He will die not knowing if he was successful or if anyone even cared. He will die by being drowned in a cave no one knew about. At least Kreacher got away and hopefully was able to destroy the locket. Unfortunately he betrayed the dark lord so when he finds out about what Regulus did there was no stopping him from coming after Regulus. He betrayed his family and friends.
Except two.
Sirius and James.
But since Sirius thinks Regulus chose to be a death eater he won't be coming to help his brother. He probably thinks it's just a trap. That is if he even opened the letter. Which Regulus hardly thinks that he did.
And James. He has Lily now. And they are with a child so James is too busy with that to even pay attention to the letter. He is probably too busy to even care what's going on with Regulus.
That is why Regulus was surprised when he started going up out of the water instead of being dragged down. Being pulled against the deathly white hands pulling him down. The second he was out of the water he saw who rescued him.
It was the dark lord and the inner circle of death eaters. Including his insane cousin Bellatrix and her husband.
Not the person he wanted to see. Especially considering he just stole from him. And betrayed him.
Regulus can't tell who is more angry with him, his cousin, who is the reason he was able to join the death eater so young, or the dark lord. But both had anger seeping out of their eyes. As the dark lord shouted "Crucio!"
The next thing Regulus knew was intense pain and agony. Fueled by the hatred the dark lord had. The next second it stopped. Leaving behind the same feeling it always did when his mother used it if he made a mistake or like when the dark lord used it if he failed, which was more often than surprised Regulus that he was still able to have control over what he did.
"Where is the locket Regulus, all this pain and agony could be over if you just tell me where the locket is."
Regulus glared at the dark lord as he told him "I will never tell you. You will lose. You will no longer be immortal. I found out about your horcruxe. It has already been destr..." Stopping as the intense pain and agony came back before he could finish speaking even longer and stronger than Regulus ever experienced.
"You idiot. You think I only made one? You are more of an idiot then you are the blood traitor of a brother Sirius. It seems the two of you are more alike than your parents thought." The dark lord said while the curse was still in effect and leaving it as he turned to the inner circle "Take him to the Lestrange manor and find out where the locket is. Once you have the locket, kill Regulus but before you kill him, break him till he is begging for death." He then turned back to Regulus crouching down to Regulus's face and whispered to him "You will regret what you did. I know you didn't destroy it, I would have felt it. You should have told me who took the locket then all this would be over and you would be dead at my feet" That was the last thing Regulus heard before he passed out from the pain.