In the Resting Abyss, High-Order Undead refers to those who possess a Soul Heart, such as Golden Skeletons, Iron Skin Zombies, Witch Kings, and Undead Saints. The even higher ranked ones include the Lord of Mourning.
But in this world, because there are too many undead, Golden Skeletons and Iron Skin Zombies can no longer be classified as 'high-order', they are considered ordinary.
The high order here refers to those that have their territory, and can suppress other undead. Every one of these beings is overwhelmingly powerful.
But, weren't the Necromancers bold enough to chase away the Undead Tide because all these top existences had gone to heaven with the king?
The Necromancers looked at each other, slowly, quietly, they turned around and fled.
When they realized a top undead creature was up front, which could suppress the Undead Tide, they panicked. That was by no means a power they could contend with.