An 18-point haul from Gareth Anscombe was not enough for Cardiff Blues, as Ospreys clinched a dramatic Judgement Day victory to seal their place in the Heineken Champions Cup play-off.
While the Ospreys opened the scoring it was the Blues that struck first with Gareth Anscombe finishing a stunning team effort.
The Wales international and Sam Davies exchanged shots at goal before former Cardiff Blues man Cory Allen claimed a try for the Ospreys, who led 16-13 at half-time.
An early try from Nicky Smith put the Blues up against it but they battled back with Josh Turnbull claiming a fortuitous try and Anscombe drawing level.
Anscombe had the opportunity to boot the Blues into the lead in the final 10 minutes but he was unable to stretch his 18-point haul.
A late Davies penalty eventually secured the victory for Allan Clarke's men, as they finish in fourth place in Conference A, keeping their European ambitions alive.
Cardiff Blues made four changes for the must win encounter with the inspirational Nick Williams returning from injury to lead the team in the absence of Ellis Jenkins and Kristian Dacey.
The other changes came in the backs with Jarrod Evans, Lloyd Williams and Aled Summerhill all coming into the starting line-up with Gareth Anscombe switching to full-back and Tomos Williams and Jason Harries dropping to the bench.
Sam Davies opened the scoring for the visitors, with a simple penalty on four minutes, but their lead did not last long as Cardiff Blues struck for a sensational try from inside their own half.
Owen Lane made the initial break as he stepped inside George North and turned on the afterburners. He found Lloyd Williams in support and he played the perfect switch with Anscombe, who skipped inside three men and held off a fourth to score.
The full-back slotted the conversion to give Wales’ Capital Region a 7-3 advantage after seven minutes at the Principality Stadium.
With the two teams evenly matched the Ospreys had plenty of territory and possession early on but Williams delivered with two turnovers – the first a textbook jackal and then a strip in contact.
Anscombe kept the scoreboard ticking with a penalty on 13 minutes after former Cardiff Blues centre Cory Allen played the ball on the floor.
Ospreys fly-half Sam Davies hit back on 22 minutes when Olly Robinson was perhaps harshly penalised for marginally straying in front of the kicker, but the Blues remained in the ascent.
A powerful carry from Halaholo should have led to points after he beat three men and Summerhill was tackled around the neck, but play resumed, and the Blues were pulled back for a forward pass at the next phase.
Wales’ Capital Region turned the ensuing scrum against the head as Robinson caught Aled Davies at the back. Mulvihill’s men were back on the attack and Halaholo threaded a kick to the corner.
Jason Harries, who had only been on a matter of minutes after replacing Lane, won the race at ease but the ball trickled into touch in goal.
The encounter took a turn for the worse just before the half hour mark when Anscombe slipped returning a kick. A dubious penalty as Nicky Smith jackalled for the ball. Anscombe complained that he was not released but the ball was fired to the corner.
Cardiff Blues resisted the first attack but when the Ospreys were awarded a free-kick they struck. They went close at first attempt and the damage was done with Allen crashing under the posts from close-range.
Davies added the extras to hand the Swansea-based outfit a 13-10 lead and it could have been more as they twice set alarm bells ringing on the counter. Thankfully the second break was created by a blatant block and Nigel Owens awarded the penalty.
Ospreys snatched the lineout but Williams rose again to claim his second turnover penalty and Anscombe levelled.
Davies had the opportunity to put the Ospreys back in front going into half-time, as Rhys Gill was pinged for a neck roll, but he pulled his effort right of the uprights. However, with a minute left was handed another opportunity and made amends.
The Blues were dealt a further blow early in the second half as the Ospreys flexed their front five muscle following a fine kick to the corner from Dan Evans.
They went through a succession of phases before Smith burrowed over and Davies converted for a 23-13 lead. Mulvihill’s men were now up against it but they were given a glimmer of hope when Cory Allen was sin-binned for a deliberate knock on.
Jarrod Evans went to the corner and although the lineout was lost a thunderous hit from Ethan Lewis on Scott Baldwin dislodged the ball. Davies and George North failed to deal with the loose ball and Turnbull burgled them before powering over.
Anscombe converted and he was soon back at the tee as the Blues struck for a quick-fire second as Evans scythed through and put Tomos Williams away. However, replays concluded that a block from Williams created the hole and the try was scratched.
That left the scores remaining 23-20 to the visitors and the yellow card period ticking away. With little more than 10 minutes remaining, Anscombe levelled the scores to set up a thrilling finale and it certainly delivered.
An Evans chip unlocked the Ospreys defence and Rey Lee-Lo collected to break through, he looked to put Tomos Williams away but Dan Evans blocked the pass and was duly sent to the bin.
In all likelihood the electric scrum-half would have made it to the whitewash but it was impossible to say for sure and Owens could only award a penalty, which Anscombe missed.