I think they have a hold of Mugo in some way and the sacrifices are part of some contract however they never counted a sacrifice no dying. By keeping Ja-Heon alive Mugo seems to be getting stronger(maybe, since they are fusing together) so the clan will probably tried to get Ja-Heon back and kill him like they did the first sacrifice(maybe, I don't know, I'm just guessing jajajaj)
I think they have a hold of Mugo in some way and the sacrifices are part of some contract however they never counted a sacrifice no dying. By keeping Ja-Heon alive Mugo seems to be getting stronger(maybe, since they are fusing together) so the clan will probably tried to get Ja-Heon back and kill him like they did the first sacrifice(maybe, I don't know, I'm just guessing jajajaj)