In Dungeon Lords, there is a spell that forces you to pay your monster(s) cost or withdraw it from combat. If you have a vampire and are just below the evil-o-meter level needed for the paladin to show up, but no one had breached the level yet, will he show up mid combat if you pay the cost to keep your vampire from withdrawing? What about if someone else had already summoned the paladin, and you became more evil than them, would doing this bring him to your dungeon?
See this paragraph on p19 of the rulebook:
The Paladin's justice is swift - if you become the most evil player at any point, he will swoop down to punish you at once! I believe I read the game designer saying somewhere that you know you've gotten really good at the game of Dungeon Lords when you are deliberately manipulating the Paladin's movements - e.g. sending him to trash another player's Dungeon, but then having him come back to yours with a few damage counters on him, so he's easier to defeat. I think it's very hard to pull off stuff like that, though!
EDIT: Regarding whether the Paladin can enter play when a player crosses the "evil threshold" for the first time in the year during a battle, I see no reason why not. Here's the rules from the Paladin section:
Yes, it's true that these rules normally entail the Paladin joining the adventuring party while they're still outside the dungeon, preparing for their attack. But crucially, there's no contradiction between the above rules and the Paladin joining mid-battle. During battle the adventurer tiles are still in the spaces near the entrance, and it's easy for the Paladin to join them. Just because in some sense the party have left the entrance and are now several tiles deep into the dungeon, doesn't mean that it's hard for the Paladin to find them.
Fundamentally, attracting the attention of the Paladin happens instantly at any point that a player becomes evil (or more evil) enough. The rules are quite clear what happens whenever this occurs, and there's no reason to suppose there should be any exceptions.