Sealed His Power
Lan was at first surprised, but he quickly understood the reason. As an Overlord, if he was allowed to roam free in the Ghost Realm, then no one would know how the world could end up.
However, if he allowed them to seal his powers, he would not be exceedingly strong, and thus he would be unable to cause much damage.
Although he did not know much about the Ghost Realm, he was very familiar with the sealing technique that the ox-headed man referred to.
This seal was not left there by the Ghost Realm itself; it would require Lan to seal his own power, which was also why he would be able to unlock it slowly after adjusting to the realm.
However, although he could put the seal on himself, he would not be able to break it. Even if he encountered a life-threatening situation, the power he had sealed away would not be unleashed. This kind of seal was the most ruthless towards oneself, but it was needed in some cases.