Dabi never admitted to having nightmares, not to anyone... except for you.
He seemed to trust you enough to let you in as to why he would sometimes be up at the wee hours of the night, or have such a stern or mildly panic-y look to his face despite his attempts at trying to mask it.
Eventually, he trusted you enough to let you help him calm down from them. He never verbally asked, usually just uttering the word 'nightmare' when he stumbled into your room half asleep at 1 in the morning.
It wasn't like he would totally freak out, just rather shakey and uncomfortable from the memories of the past. Even then, there were still times where he had rather bad ones and ended up almost ending up having a panic attack.
This night was no different. He had stumbled into your room and just layed himself beside you, and once you woke up from his presence and the movement of the bed.
"...nightmare..." The infamous line came from him as he layed there, a pensive look to his face as he kept his gaze averted.