It's... the energy made from human emotions. Demons consume it to get stronger.

[He wraps his hand slightly around Yurlungur, as though in warning. The demon coils slightly around his arm, looking at him with large eyes.] But you're off limits, Yang. Got it, Yurlungur? Don't even think about it.

[None of his demons is allowed to try and eat one of his friends, damn it.]