Swansea City reaffirmed their recent domination of the south Wales derby by beating Cardiff City to avenge defeat in the reverse fixture this season.
The Swans seized control from the first minute and deservedly led when Liam Cullen coolly volleyed in from Kyle Naughton's cross; a rare moment of calm in a febrile atmosphere.
Cardiff were utterly woeful during the first half and fortunate not to have been reduced to 10 men when Yakou Meite appeared to headbutt Harry Darling.
The visitors were given another reprieve early in the second half when Cullen rolled his penalty wide after Ronald had been dragged down by Perry Ng.
Swansea struggled to play with their earlier conviction from that point and, although they seemed anxious dealing with Cardiff's late attempts to conjure an equaliser, the home side secured a fifth win from six south Wales derbies as Jamal Lowe scored their second deep into added time.
The result lifts Luke Williams' side to 14th in the Championship table - eight points clear of the relegation zone before Saturday's later games - while Cardiff remain 11th, five points adrift of the play-off places.
Swansea's jubilant celebrations at the final whistle demonstrated how much this game means to everyone connected to the club, and how significant a lift for morale this victory could prove towards the end of what has been a turbulent season.
As for Cardiff, this was a demoralising way to end a four-match winning run in the Championship - and a deflating reminder of how poor their recent record had been in this storied rivalry.
Neither team had managed a league double in the first 111 years of this fixture, before Swansea claimed the first in 2022 and repeated the trick last season.
This time, Cardiff were hoping to make their own history having outplayed their arch rivals to win the reverse encounter 2-0 last September.
That was the Bluebirds' first south Wales derby triumph since 2021, ending a run of four successive victories for their opponents.
And while that may have been six months ago, the pain of that defeat seemed to fuel a ferocious start to this encounter from Swansea, who flew out of the blocks.
The hosts earned two corners inside the first minute, the second of which led to a powerful left-footed shot by Matt Grimes which Nat Phillips did well to divert with his head.
With the home crowd roaring them on, Swansea applied heavy early pressure with Cullen firing wide and Darling heading against the bar from Jamie Paterson's fine cross.
Cardiff were rattled - a pale imitation of the side which beat promotion contenders Ipswich Town so impressively a week earlier - and they were fortunate not to be reduced to 10 men in the 17th minute.
Darling pushed Yakou Meite to the ground as they tussled for the ball and, after the Bluebirds forward got to his feet, he leant his head into the Swansea centre-back's, but the outcome was a yellow card for each player.
Swansea were incensed, which only added to the intensity of a performance already brimming with it.
They got the goal their dominance deserved when Roland drove from his own half and towards the Cardiff penalty area, before laying the ball off to Kyle Naughton.
His cross then sailed to the back post, where Swansea academy product Cullen neatly volleyed in to prompt euphoric celebrations.
Cullen had a golden opportunity to enhance his hero status five minutes into the second half when Ronald was pulled down by Ng, but the Swans forward pushed his low left-footed spot-kick wide of the left post.
Having been well on top until that point, Swansea seemed to waver a little after that penalty miss, whereas the hitherto non-existent Cardiff sensed just a glimmer of an opening.
Bluebirds manager Erol Bulut went for broke with this substitutions, bringing on all the attacking options on his bench - Aaron Ramsey, Rubin Colwill, Callum O'Dowda, Ollie Tanner and Famara Diedhiou, but the visitors still struggled to create genuine scoring chances.
There was a desperation to their play and, the more players they threw forward, the more vulnerable they were in defence
That led to the decisive blow as Swans substitute Lowe broke into the Cardiff penalty area, evaded goalkeeper Ethan Horvath and fired into the net to seal another memorable derby day for Swansea.
Cardiff City: Horvath, Ng, Goutas, Phillips, Wilson-Esbrand, Siopis, Wintle (C), Turnbull, Bowler, Grant, Méïté. Subs: Turner, Romeo, Collins, Ralls, Ramsey, R. Colwill, O'Dowda, Tanner, Diédhiou.
Swansea City: Rushworth, Darling, Allen, Grimes, Paterson, Tymon, Płacheta, Cullen, Wood, Naughton, Ronald. Subs: Fisher, Fulton, Cooper, Patino, Lowe, Key, Yates, Abdulai, Lissah.