Scarlets completed a United Rugby Championship double over Cardiff with a stunning seven-try win.
Cardiff took an early lead through tries from Theo Cabango and Lloyd Williams before being humbled at home.
Two tries from Liam Williams and further scores from Aaron Shingler, Dan Davis, Tom Rogers, Jonathan Davies and a penalty try handed Scarlets the victory.
Scarlets' win keeps alive their hopes of reaching the play-offs.
Sorry Cardiff capitulated at the Arms Park as they conceded three tries in the last 10 minutes and shipped 42 second-half points in front of their own fans.
Dai Young's men had previously been hampered by second-half yellow cards for Jarrod Evans and Seb Davies.
Scarlets have risen to ninth in the table after a hat-trick of bonus-point wins and will be aiming to finish in the top eight, while they are in pole position to finish as the top Welsh region.
There were, though, concerning scenes for Scarlets and Wales as prop Wyn Jones and full-back Johnny McNicholl were forced off the field in the second half.
Cardiff crowd
A Saturday afternoon kick-off in front of close to a Cardiff capacity crowd between two Welsh sides gave a glimpse of what thriving domestic rugby could look like.
With no Welsh regions achieving Heineken Champions Cup last 16 qualification, this was an opportunity to reschedule this rearranged match that had been a victim of Covid-19.
Scarlets had taken the spoils in the opening 35-20 win seven days before and made five changes, including the intriguing selection of wing Liam Williams, who will join Cardiff next season.
Williams was celebrating his 31st birthday and making only his seventh appearance for Scarlets since re-joining from Saracens in 2020.
He had also been sent off in this fixture last season for an illegal ruck clearance on Cardiff flanker Shane Lewis-Hughes, who had recovered for this match from injury.
British and Irish Lions star Williams went on to to show Scarlets what they will be missing next season and what Cardiff can look forward to.
Lewis-Hughes was one of 10 changes made by Dai Young who had given his squad a stark pre-match warning about their futures.
There was a late change with captain Josh Turnbull ruled out in the warm-up through injury and Will Boyde slotting in at number eight.
Cardiff demonstrated they had heeded the words of their boss with a third minute try started by the dancing feet of full-back Matthew Morgan.
It was clinically finished by Cabango on the other wing as he glided outside McNicholl and Davis. Evans converted from the touchline.
Cardiff doubled the lead after Morgan again started the counter-attacking move before returning wing Owen Lane released Lloyd Williams to canter away from the despairing attempted tackle from Sam Lousi.
Evans converted before missing a penalty, while Scarlets lost hooker Ryan Elias and lock Jac Price following a clash of heads.
Kalamafoni, who had been cleared to play despite being sent off for a high tackle last weekend, was denied a try following a quick tap penalty.
Scarlets scored soon after following an incisive break from impressive captain Jonathan Davies laid the platform for Shingler to power through a Morgan attempted tackle just before half-time to cut Cardiff's advantage to seven points.
Scarlets' revival continued when McNicholl glided outside Lloyd Williams to create an early second-half try for Liam Williams with Costelow converting.
McNicholl was injured in the process of creating that try, while Cardiff fly-half Evans was given a yellow card for a high tackle with his shoulder catching the head of Scarlets centre Johnny Williams.
Lock Seb Davies followed Evans into the sin-bin for deliberately collapsing a maul with referee Adam Jones awarding a penalty try.
Scarlets took advantage of Cardiff being reduced to 13 men with a bonus-point try for Dan Davis, who immediately made way for the returning Josh Macleod.
British and Irish Lions prop Wyn Jones was forced off the field after coming on as a replacement, while Cardiff hooker Kirby Myhill received a heavy head injury before thankfully walking off the field.
Due to injuries to Ellis Jenkins and Will Boyde, Cardiff were forced into a late back-row reshuffle with centre Garyn Smith packing down at flanker and it showed.
Williams capitalised as he sliced through the Cardiff defence to grab his second try.
Rogers and Davies took advantage of Cardiff completely shutting down with late tries to complete the hosts' humiliation.
Cardiff Rugby: Morgan; Lane, Lee-Lo, G Smith, Cabango; J Evans, Lloyd Williams; Carre, Myhill, Arhip, S Davies, Screech, Lewis-Hughes, Jenkins (capt), Boyde.
Replacements: Belcher, Domachowski, Davies-King, Teddy Williams, Boyde, Tomos Williams, Priestland, Summerhill.
Scarlets: McNicholl; Rogers, J Davies (capt), J Williams, Liam Williams; Costelow, G Davies; S Thomas, Elias, Sebastian, Lousi, J Price, Shingler, D Davis, Kalamafoni.
Replacements: S Evans, W Jones, O'Connor, Tuipulotu, Macleod, Hardy, O'Brien, J Roberts.
Referee: Adam Jones (WRU)
Assistants: Rhys Jones & Carwyn Williams (WRU)
TMO: Sean Brickell (WRU).