Cardiff claimed a first victory over Leinster in 11-years as Jarrod Evans showed nerves of steel to slot a dramatic last gasp penalty.
The visitors were ahead at the break after a stunning Owen Lane effort gave them the advantage in front of an electric returning crowd at the Arms Park.
Leinster battled back, thriving when Cardiff were reduced to 14 men due to a James Botham yellow card, but Hallam Amos' try gave the hosts hope.
With the clock in red, Evans opted to go for a difficult penalty effort but kept his cool to send the Cardiff crowd into frenzy.
It did not take Leinster long to boot the scoreboard into action as they were awarded an early penalty, which Ross Byrne slotted.
Leinster enjoyed plenty of early pressure and a five-metre scrum laid the platform for an ideal assault at the Cardiff try-line but Corey Domachowski had other ideas as he drove through Michael Ala’alatoa to earn a penalty.
That allowed the Blue and Blacks to gain some territory but they struggled to cope with the Leinster rush defence, which seemed to stray forward a little early, in the opening stages.
Leinster enjoyed the next real foray and again it looked ominous for the hosts but Dmitri Arhip channeled his inner back-row to earn a textbook jackal penalty.
Jarrod Evans levelled the scores on 19 minutes, after a rampaging rolling maul was dragged down by the visitors and the fly-half orchestrated the opening try just minutes later.
The opportunity began after Aled Summerhill tapped back a monstrous Lloyd Williams up-and-under to catch the Irish defence off guard. Evans spotted their rush defence and space out wide, opting for a cross kick, which was weighted perfectly for Owen Lane.
Lane took the ball in pace, and oozing confidence following three tries in his last two games, he had one thing in mind as he went on the outside and scorched past Rory O’Loughlin and Jimmy O’Brien for a fine score.
Evans added the extras to give his side a 10-3 lead but Leinster instantly hit-back as they exposed a slight disconnect in defence. Ross Byrne was able to offload out of contact and Adam Byrne cut a razor-sharp line to slice through.
Byrne levelled the scores but in an all action period, which saw Evans depart for a HIA, Ben Thomas nudged Cardiff back into the lead.
Leinster, showing all of their champion class and experience, threatened every time they had the ball thanks to their intensity and accuracy on the ball.
Cardiff defended admirably and forced a knock-on with the try-line beckoning, whlle they repelled another sustained attack thanks to a Will Boyde turnover penalty.
And it was case of Boyde at the double as he claimed another jackal to win a penalty as the clock ticked towards red.
Cardiff entered the half-time break, which saw a presentation to the semi-professional players who helped the club during the Heineken Champions Cup honoured, with the slender 13-10 lead.
Evans back on the field quickly stretched that to 16-10 and a rampaging break from Sam Moore, who burst out of his own 22, saw the Blue and Blacks launch a sustained period of pressure.
The 25-year-old soon added a fourth penalty to make it a nine point lead before Leinster came roaring back. They kicked a pair of penalties to the corner as they kicked a pair of penalties to the corner.
It looked as though Cardiff had survived unscathed as Leinster were held up over the line but after consulting the TMO, Mike Adamson sent James Botham to the bin for hands in the ruck and gave the Dubliners another penalty.
They took the tap and after a succession of phases James Tracy wriggled over. Byrne converted to cut Cardiff’s lead to just two points.
A sickening clash, which saw Rory Thornton knocked out and receive lengthy treatment, halted the game for an extended period and just moments later the home side were penalised for a high tackle.
Leinster again kicked to the corner and Scott Penny raced over from a set-play to give his side the lead, which Byrne converted to 19-24.
As the 70 minute mark approached Harry Byrne hit the uprights with an effort from wide, which Rey Lee-Lo collected and moments later Aled Summerhill was tearing down the left.
He haded off Adam Byrne but lost his feet as he battled to stay upright. However two phases later he looped around Evans and offloaded out of contact for Amos to claim a stunning try.
Evans converted from the right touchline to claim a two-point lead but Leinster soon snatched it back thanks to a penalty.
Leinster replied with a Byrne penalty and seemed to have the win in the bag, but with the clock in red, the hosts kept coming. They were rewarded for their persistence as the visitors were penalised for a high tackle.
40 meters out and to the wide left of the field, Evans kept his cool to slot a stunning effort to secure a momentous victory for Young's men.