Ferdinand and Raymond also wanted to speak with the Pope about the undead which was why this meeting was agreed upon.
"I believe you must maintain Valeria for a long while. With the number of your knights and soldiers, the undead is more dangerous to your Empire than several kingdoms altogether," the Pope added.
This was true as well.
Several kingdoms attacking together could be won over with their might.
On the other hand, the undead only followed the command they were given by the necromancer. And they keep reanimating unless dealt with properly to stop them completely.
With their numerous knights and soldiers, their numbers could be turned against them, since any death on their side becomes the strength of their enemies, who use necromancy.
Even if their army was strong, they would get tired eventually, while the undead were unfeeling and tirelessly fought continuously.
Thus, they indeed needed to ensure that using necromancers in war doesn't become a thing.
