Corey Domachowski demonstrated the impossible match-champ as he grabbed an emotional last-wheeze triumph at Rodney March Rugby.
JJ Hanrahan, the flyhalf for the Dragons, believed he had ended their 14-game losing streak to their local rivals.
In the second half of a close derby, Harrison Keddie had followed Rio Dyer over the try line as Dragons recovered.
However, Josh Adams’ try was followed by two tries from Tomos Williams, and substitute prop Domachowski barged in 30 seconds later for a thrilling victory.
Despite Dragons’ bravery and honesty in their performance, this defeat will not be lessened.
They had a win with real significance in their hands, but Cardiff were rewarded for their bravery and clinical play in the end.
Domachowski scored from a line-out five meters out after Jarrod Evans converted a penalty kick half the length of the field.
Cardiff moved up to fifth place in the United Rugby Championship thanks to the outstanding performances of scrum half Williams and back row Taulupe Faletau under Gatland’s watch.
In addition, they will be able to brag about these local accomplishments in front of the returning Warren Gatland, the new Wales coach.
Gatland will have been given food for thought by Wales hopefuls Evans, Thomas Young, Rhys Carre, and Dragons wing Ashton Hewitt.
Back Adams, on the other hand, will be a concern after limping off injured, and Dragons lock Will Rowlands announced prior to kickoff that he will not participate in the Six Nations.
Flanker Young put on another smart performance, Carre scrambled and carried well to score a try, and Evans took over at number 10 to score three more tries.
Cardiff’s finishing was the difference as they scored two tries in three minutes to take a 14-11 lead at halftime against the flow of play.
Mythical beasts overwhelmed the initial trades yet neglected to underwrite.
Adams was moved from wing to fullback because Liam Williams is still a week away from returning.
In Bradley Roberts’s first Boxing Day derby, a ferocious trip off the ball earned him a yellow card, kicking off a fierce contest.
Anyway 14-man Winged serpents actually figured out how to take a noteworthy lead through the boot of Hanrahan.
After missing the autumn Test series, Ross Moriarty was in the thick of the action, both on and off the field. He had a point to prove.
Faletau, his opponent and Wales rival, was, however, in a different class.
After 33 minutes, Cardiff finally got their attack going, and the number eight appeared everywhere.
Prop Carre, who was also trying to get back into the Test team, was involved twice, giving Williams the scoring pass with a well-timed offload.
Priestland limped off with a lower leg injury yet substitution Evans promptly kicked the change and before long added one more subsequent to setting up a second attempt soon.
Adams was able to touch down for a 14-6 lead outside thanks to his deft inside pass to Lane.
Mythical serpents were staggering however hit back before half-time. Moriarty conveyed and Steff Hughes’ flung pass gave Dyer a simple completion out wide.