Cardiff City halted Swansea City's run of four successive south Wales derby wins and claimed their first since 2021 as Ollie Tanner came on to change the course of the match with an inspired substitute appearance.
Despite a frenzied atmosphere at Cardiff City Stadium, a staid first half produced only one shot on target for the home side's Yakou Meite.
Cardiff were much the better team as the game came to life after the break and, after Meite was twice denied by Swansea goalkeeper Carl Rushworth, they finally got their reward when winger Tanner came on and, seconds later, cut inside on to his left foot before rifling a ferocious shot into the far corner.
The player signed from non-league Lewes last year then earned a penalty as he was dragged down by new Swansea signing Kristian Pedersen, and Wales captain Aaron Ramsey applied a nonchalant finishing touch as he sent Rushworth the wrong way with his spot-kick.
While the Bluebirds rejoiced in only their fourth derby victory in 12 attempts - and their first at home for a decade - a fourth defeat from six leaves Swansea in the Championship's bottom three as head coach Michael Duff still searches for his first league win.
Although this was the 69th league meeting between the teams, it was not until the 2021-22 season that either team had managed to do the double, with Swansea repeating the trick the following campaign.
During that period under previous head coach Russell Martin, the Swans regularly outplayed their relegation-threatened rivals, but, with Cardiff starting this season more promisingly under Erol Bulut than Swansea had under their own new boss Duff, there was a sense before this match that the tide could be about turn.
So it proved as Cardiff turned their dominance of a hitherto forgettable encounter into a night to remember for their fans thanks to Tanner's dazzling intervention.
Duff had incurred the wrath of some Swans supporters before this match by saying he would "rather win promotion and get beat by Cardiff twice" than win his first derby in charge, only to then backtrack on those comments.
In response, Bulut said - with a glint in his eye - that he was targeting promotion and two wins over Swansea, pointing to his experience of managing Fenerbahce in hostile Istanbul derbies as a way of understanding this fixture's significance.
This contest did not quite register on the Galatasaray 'welcome to hell' end of the derby scales, but there was an unmistakable air of intimidation about a Cardiff City Stadium close to capacity.
Yet while the atmosphere hissed with intensity, the game itself took some time to catch fire.
Both teams probed at each other carefully in the first half, Cardiff enjoying more possession, but struggling to create real scoring opportunities while Swansea were limited to occasional long-range efforts.
The only proper chance came with almost the final kick of the half as Ike Ugbo skilfully headed a lofted pass from Ramsey into the path of Meite, who turned and had a low shot well saved by Rushworth.
It took the second half less than 30 seconds to match the first for action, as Rushworth had to tip Meite's fierce 25-yard strike over the crossbar.
The Swansea keeper thwarted Meite again soon afterwards and, with Cardiff firmly in the ascendancy, Karlan Grant was the next to threaten as his shot was blocked by Jay Fulton.
The game changed when Tanner was introduced. From the first phase of play after he came on, he controlled a cross-field pass by Jamilu Collins, shifted the ball on to his left foot and fired into the bottom far corner.
Swansea could not handle the former Lewes winger, who ran directly at his opponents whenever possible and drew a clumsy foul from Pedersen for Ramsey's penalty.
The Swans had some half-chances as the match entered its final stages, but they were never in contention, and already former Barnsley and Cheltenham Town boss Duff will face questions after a concerning start to his reign.
This night belonged to Cardiff, though, and their jubilant celebrations which greeted the final whistle at a rain-soaked and rocking Cardiff City Stadium demonstrated just how much it means to win this famous old fixture.