I think it's to convey the severity of the power gap. Calder is probably going to be quite a challenge for the group, which means they're in turn not necessarily ready for Team Evil. If they can't readily crush an old red dragon with lots of power and domination magic at his disposal, how are they going to take on an Epic sorcerer, a cleric with 9th tier spells, and an unknown creature with enough raw power to make earthquakes at will?

I think fighting Calder is going to give Serini reason to doubt the ability of the Order's success, even if she has no choice but to stick with them now that the threat of the gods' annihilation is on the table and she can't just let any team win without conflict any more.