"A-Yuan, shh, shh, it's all right," Wei Wuxian soothed, half-panicking and torn between terror that Wen Qing was going to come into the cave any moment, see A-Yuan like this, and even more terror because – well, A-Yuan was hurt.
The rational part of Wei Wuxian's mind told him that he was overreacting: head wounds often bled profusely, and a brief examination of the side of A-Yuan's head had shown that while blood was running down his face, the cut was only a superficial wound. The rest of Wei Wuxian was busy being guilt-stricken at letting A-Yuan run wild inside the Demon-Subduing Cave where his scattered writings and experiments were hazards in a place where great streaks of rock jagged out of the ground and from the walls. A slip of the feet, a headfirst lunge near the Blood Pool, and there was A-Yuan, crying as though his little heart would break.
Wei Wuxian's heart was going to break at any moment too. He mopped the blood with a none-too-clean rag, for once bitterly regretting giving up his golden core because he needed it to have spiritual energy to heal A-Yuan, damn it.
Why the fuck couldn't resentful energy-
Well, Wei Wuxian reasoned, resentful energy was certainly used for destructive purposes, but there was nothing to say that it could not be re-purposed. Right? He was the Yiling Patriarch, the current expert in all energy resentful. He could do this. Decision made, he coaxed, "A-Yuan, A-Yuan, shh, Xian-gege is going to try something, all right? Hold still for a moment."
He was sure that even if this didn't work, it wouldn't harm A-Yuan. Well, he was mostly sure. With the way A-Yuan was crying, he had to do something.
A short whistle called a soupçon of resentful energy into his hands, quiescent only because of his control. Visualizing what he needed (once upon a time, he had used spiritual energy for healing), Wei Wuxian guided resentful energy into the cut.
There was a pause as resentful energy pulsed, as though alive, and then it worked: the bleeding stopped, the cut gradually sealed together and soon there was nothing of the cut except a faint pink line.
A few minutes passed before A-Yuan realised belatedly that the pain had disappeared, and his sobs dried up, as he sat blinking big doe-eyes at Wei Wuxian in bemusement. "Not hurt anymore," he informed Wei Wuxian.
"Yup, Xian-gege made the hurt go away," Wei Wuxian mumbled, relief and terrified triumph mixed together. He mopped the remaining streaks of blood from A-Yuan, pinching a cheek in affection. "Don't tell Wen-gugu."
After a moment's thought, A-Yuan nodded. Wei Wuxian examined him again, seeing a properly healed scar that was going to be covered by his hair, and allowed himself to make a fist of victory.