"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

Build the "Temple of the Underworld" and turn anyone into undead. To clarify: go to the castle, on the left there's a panel with buttons for the tavern, factory, etc., and in the center, there's a large button – press it, and it will transform creatures from other factions into undead. That's it!

Oh, thanks, bro, I was messing with the "central" button and couldn't figure it out))))

Added 3 minutes later
And here's another question:
When you win, a list of creatures appears at the top, which are supposedly raised from corpses into undead, with the possibility of upgrading them and adding them to your troops using dark energy.
Sometimes you can upgrade two squads and move them to your side, and sometimes you can upgrade them, but you can't move anyone.
What determines this?