"They could've sent an army to raze all the clans of this Realm. However, before the limitations of this world, their army couldn't recover their strength once they used it. At the same time, the Blood Clan could recover nonstop by absorbing the blood of each person they killed."
"The Blood Clan could fight nonstop as long as they could kill enough. In other words, every person they killed became their strength! That's why, the Upper Realm gave up and just forbade their people from entering this realm."
"The Upper Realm doesn't send anyone because there's no benefit and only death that awaits their people if they come here. However, at the same time, the Nether Realm can't send anyone outside either."
" Their advantages are essentially useless in the outside world where people can recover their strength from their surroundings. That's why the stalemate took place."