The warrior mare Tek taking on the seven-headed wyvern king, who is many times larger than normal wyverns (and wyverns being many times larger than unicorns) in one-on-one combat...and killing three of his heads.
The unicorn princess Lell throwing a flaming brand into the eyes of the wyvern king's main head, not only blinding him but trapping him in a ring of fire.
Lell becoming trapped in the fire and setting ablaze, when the gryphon Illishar swoops down and carries her to safety, despite the fire engulfing his body and wings while he's at it.
The arrival of the Scouts of Halla, who (thanks to the mystical dragonsup) made a weeks-long journey without tiring to join the battle just as the tide had turned in the wyvern's favour.
The unicorn prince Jan finally arriving to the field of battle...in the claws of the dragon queen.
The dragon queen hauling the wyvern king out of the fire - six of his heads dead, the seventh blinded by fire - and carrying him back to the dragon's homeland to be a plaything for her future mate.
Jan trampling his way through the wyvern's cavern, his hardened hooves setting a blazing trail all throughout the Hallow Hills.
The titular Endingfire being a blaze so great that the rain of soot and ash blocks the sun for three entire days before being washed away in a great rain.
Arguably, when Jan discovers how he can create fire in Dark Moon while trapped on the edge of a cliff.
And then, there's the fight against the wyvern queen in the first book.