Cardiff fell to a second consecutive defeat to the Scarlets after discipline and injuries took a toll in a significantly disrupted second-half.
The Blue and Blacks made a dream start to the United Rugby Championship encounter with tries from Theo Cabango and Lloyd Williams.
Scarlets kept themselves in the encounter with a try from Aaron Shingler in the final play of the half and after Liam Williams crossed early in the second they showed no mercy.
Decisions went against the hosts, who last Jarrod Evans and Seb Davies to the bin, and a series of injuries saw numerous players utilised out of position.
Scarlets claimed three more tries thanks to the whistle of Adam Jones, a close-range Dan Davies efforts following endless replays, and a late second for Williams.
Tom Rogers and Jonathan Davies extended their lead in the latter stages, capitalising on errors by the hosts.
The defeat leaves Cardiff chasing against the odds in both the United Rugby Championship table and the Welsh Shield as attention turns to Europe.
Cardiff made a flying start to the afternoon encounter with Matthew Morgan instigating the first real attack of the day.
After receiving a kick on the edge of his own 22, he dummied a clearance before launching the counter, he skipped past two defenders and fainted a pass inside. He then offloaded to Owen Lane on the outside, who did remarkably well to stay in play.
Possession was maintained and the ball was now spread to Morgan on the right, who fed the scoring pass to Cabango. The young wing had plenty to do but beat his opposite number and then showed enough pace to outstrip the cover for his fourth try in as many URC games.
Evans bisected the posts from the touchline and while the Scarlets also threatened in a fast and furious opening quarter, Cardiff soon stretched their lead with another fine try.
Owen Lane deserved plenty of plaudits for working tirelessly on a kick-chase and then tracking down Costelow. The Scarlets cleared but fine work from Evans and then Morgan once more put the home side on the front foot.
Then when the ball was spread to the right, Will Boyde drew a defender and released a clever offload out the back to give Lane half a chance. He unleashed a devastating step to stand up a Jac Price and burst down the outside. He then drew Johnny McNicholl to put Lloyd Williams away.
Evans added the extras to open up a 14-0 lead but he was off target on 25 minutes following some more uncompromising work from the pack.
A series of kicks to compete around the half hour mark almost unlocked the Cardiff defence as incoming summer signing Liam Williams twice won the ball in the air. The ball was spread right and after the Scarlets passed inside a try looked inevitable but Evans appeared like a bolt from the blue to clatter Gareth Davies.
The Scarlets went through the phases as they attacked the five metre line but Seb Davies stripped the ball from Carwyn Tuipulotu to save the day and Lane cleared long.
Dwayne Peel’s side controlled the final minutes of the half as they kicked a succession of penalties to the corner. Cardiff defended well but it was the visitors who benefitted from the decisions of referee Adam Jones.
Following a third penalty, the Scarlets went quick and Sione Kalamafoni appeared to cross but replays showed he knocked on. It seemed as though Cardiff has survived but the Scarlets attacked through injury time and Aaron Shingler eventually crossed in the corner with Costelow converting.
While Cardiff made a fine start to the first half, it was the Scarlets who started the second better and some clever footwork from McNicholl, following period of pressure, created space to put Williams away.
Costelow was again on target to level the scores and matters were soon made worse when Jarrod Evans was harshly sent to the sin-bin.
It was an unfortunate chain of events as Johnny Williams fell into Evans’ tackle with the fly-half making contact with the head. With a starting point of a red card, there was more than enough mitigation to downgrade to a yellow but referee Jones had no other option in the current framework.
The Scarlets kicked to the corner and turned the screw with their sizeable pack and were eventually awarded a penalty try with Seb Davies joining Evans in the bin.
With Cardiff now reduced to 13 they had to weather a significant storm and the Scarlets soon claimed a bonus-point try through Dan Davis, despite an eternity of television replays.
Costelow kept the scoreboard ticking and while Cardiff showed plenty of fight, the decisions continued to go against them with the penalty tally now reading 14-5 to the west Walian’s favour.
Cardiff simply could not catch a break with Ellis Jenkins forced off and Kirby Myhill utilised in the back-row with Liam Belcher now at hooker.
If that seemed bizarre enough, Myhill soon departed after being knocked unconscious, leading to Garyn Smith packing down at six with Aled Summerhill the only remaining replacement.
Williams claimed his brace to take Scarlets out of sight, before Rogers and Davies both added their names onto the scoreboard, taking advantage of Cardiff errors.
The Blue and Blacks will look to bounce back when they turn attention to next week's Challenge Cup knockout clash, with the opponents to be confirmed later this evening.