Arambrough was locked up, and until he figured out why, there was no possibility of him being released. To his relief, Ange had also released the twelve Undead Cavalry under his command.
The Undead Cavalry were also shaped from the undead, and in essence, were undead. The difference from the Black Warriors lay in the fact that they hadn't been solidified through the Reincarnation Altar; all their weapons and armor were merely Soul Armor, making their melee capabilities quite weak.
But there was no helping it, not all undead could learn magic. Some personalities simply weren't suited for magic; they couldn't settle down. Therefore, Anthony had tried to improve the Reincarnation Altar, thereby creating a new type of undead creature—the Black Warrior.
However, Anthony originally had no Soul Contact with the Undead King, meaning His Majesty didn't know about the Black Warrior transformation. The altar at the Master God's Temple was merely for the ordinary reincarnation of the Undead.