Transformation of undead in the Temple of the Underworld.

Scripts cannot be bound to interface buttons; they can only be bound to script dialog boxes or triggers. As for modding tools... I'm not sure. If you connect an NCF file, you can create duplicates of the creatures you need, but with a different level. When entering the Necropolis, replace the normal creature with the duplicate; when entering a battle or leaving the city, switch it back (to preserve the game mechanics tied to the creature's level). Create the new creature using modding tools and change it with scripts. Although, as always, I'm just generating complex solutions.