Sorry, I had to. Lucifer felt Balthazar's Grace when he was still in the Pit, knew he was still alive when they told him he wasn't.
Basically the sequel to "Clipped Wings to Bring Us Together."
He felt it. He knew he would, of course, but knowing and actually experiencing it were two completely different things. He knew how it would feel, too, since he'd felt the Grace abandoning Gabriel's body. The universe probably felt that event, an earthquake that rocked every molecule of matter, a shockwave rippling through the cosmos. Archangels don't die without people noticing.
But this was different. Balthazar wasn't an archangel. He was a low-ranking angel, and as they went, barely worth the gust of wind from an archangel's wings. No, in the ranks of the Heavenly Host, Balthazar was no one. Just a face, a pawn. Practically beneath notice.
When they told him Balthazar had died the first time, he reached out with his Grace for that tiny spark that he'd recognize anywhere. He found him, hiding from Heaven among the humans, keeping himself safe. Good angel, he'd thought, smiling to himself. But on the outside, he stayed calm, as though news of this random angel death meant nothing to him. And he pretended for millennia, pretended he wasn't secretly tracking Balthazar, making sure he knew where he was.
And then Sam Winchester, ble—da—whatever his soul, sprung him from the Pit. And yes, jump-starting the Apocalypse was his main goal. But he knew he had to see Balthazar again. And even after Balthazar gently reminded him that his "no" still stood, he let him go. He couldn't possibly kill Balthazar, not after waiting so long to see him again.
And then, before he really knew what happened, he was back in the Pit, thanks again to Sam Winchester. After that, he knew—he didn't want to believe it, but he knew—that he would never see Balthazar again. At least he gets to stay safe. It made his punishment that much easier to bear. Having Michael and Adam and Sam there to rough up with him helped, too. He was able to vent a lot of frustration, especially on the latter two. He and Michael called a temporary truce to punish their treacherous vessels, calming him down.
When Crowley came along and snatched up Sam, he just attacked the soul that Crowley had conveniently forgotten. When Death came by a year later and picked up the soul, too, he and Michael were left with just Adam.
But it wasn't that long after when he felt it. It was maybe hours—Sam may not have even had his soul back when it happened. He felt that cataclysmic shockwave, and the worst part was that he was probably the only one who felt it. He was the only one who knew the life, the Grace was gone from his Balthazar. Only he and the person—whoever they were—who killed him. If he ever got out of the Pit again, the first thing he would do is find Balthazar's killer and make them suffer.
But he was still stuck in the Pit right now. Trapped with Michael and Adam, neither of whom were aware of any significant event. The last person who truly loved him, who showed him any sort of affection, was dead.
Lucifer threw his head back and howled, and the demons standing guard over the Cage trembled.