Galen and Valerian's father work on the spear The Blacksmith created as a weapon against the dragon Vermithrax. When Tyrian — fulfilling The Dragon trope — tries to stop Galen from rescuing Princess Elspeth, Galen uses that spear to kill Tyrian.
One better according to the novelization: The spear was specifically enchanted to kill dragons. Galen is lacking in combat skills, but the spear itself was drawn to the dragon symbol on Tyrian's chest.
The spear was made to kill Dragons. Tyrian was The Dragon.
Some complain that if Galen simply had sex with Valerian, she'd be spared from the Lottery. However, "virgin" in the Middle Age setting context may just mean a woman below a certain age, since if being a virgin was a qualification, it's pretty obvious other girls in the village would have thought of that escape clause far earlier. Besides, Vermithrax is not a god - she is a mother feeding her young and herself. Doubtful she'd care if the food had sex or not.
Though in turn, it becomes bizarre that Vermithrax only wanted young girls.
If only girls are served to it... Vermithrax probably thought the priest was a nice meal, too.
Also, guess how young the girls would be if older teens did have sex? Would you rather offer up a teenager or a little girl?
Fridge Brilliance: the horse that shows up at the end wasn't magic. It was the horse from the wagon carrying the first girl to be sacrificed. It was wandering around, avoiding the dragon, and came out when it saw some friendly looking people!