It's the indifference that makes him both angry and edging onto fear. Lucifer did warn him about how getting closer to his power would make him, it isn't unknown, but it's still a shocking difference with what he's used to. Lucifer should be annoyed and snarling at him, feeling defensive and blaming everyone but himself, convincing himself he was right while some part of him feels guilty.

The distress Istredd feels is as intense as his anger followed by deep internal pain. Has he already lost him? Was this taste enough for him? Istredd would do anything for Lucifer to get pissy at him and throw one of his occasional tantrums.

"Don't." Istredd steps into him, gripping his face firmly between his hands. Blue eyes are intense and searching his gaze and their bond flares up like a torch. His emotions have always been strong, sometimes too strong for Lucifer to deal with comfortably, and he's going to remind him of that. His anger and sadness mix with fear now, but also with stubborn overpouring love that he's never hidden once it was revealed.

"Don't go where I can't reach you. Lucifer, please." He warned Istredd but he also told him to hold on and try anyway. "I love you even when I hate you, don't forget that." He may have brought a danger to the Nether and its people if Lucifer keeps going down that road. It's not a good feeling.