Gotta say, one of my favorite scenes in the show/light novels is when [SPOILERS]

all that buildup and all that payoff is just so incredible. the army coming from nowhere to blindside both the reader and Prince Barbro's army in the nick of time, just for them to thoroughly wipe the floor with them, the way literally everyone is shocked by this genuinely random development (that still makes total sense in the context of the story) just grabs me like nothing else. it's not something this story does often either, so it's especially special (ie it's not overdone in the story so it isn't stale). seeing the underdog suddenly get a huge set of reinforcements is crazy n I love it so much. and since it's a side character, it's an excellently well placed foil to Ainz already having that power and defeating the rest of the kingdom's armies. it's just... chef's kiss