Hu Xiao from his forehead exuded a drop of nearly black concentrated demon blood, Su Chengyu similarly forced out a drop of essence blood from his own body to merge with it.
At the moment when the two bloods fused, Su Chengyu simultaneously established a pact. After Hu Xiao's demon Qi merged, the essence blood transformed into two marks and shot into their foreheads, subsequently falling into their Sea of Consciousness Spiritual Platform.
As long as the terms of the pact are not violated, this essence blood will not affect either of them.
However, once violated, it will retaliate against its host, though it will not be lethal.
At least at the moment of violation, the violator will suffer backlash from all their mana or demonic power within, losing all abilities to act temporarily, and it only has a one-time effect.
In terms of restraint, it is indeed not very strong, but it sufficiently meets Su Chengyu's needs.
