Over 30,000 fans saw Welsh Lions like Alex Cuthbert, Jonathan Davies and Taulupe Faletau grace the Cardiff turf.
Cardiff Blues ended a run of eight successive defeats to the Scarlets with a fiery 17-13 win, which saw both teams end with 14 men.
Cuthbert put the Blues in front when he crashed over near the corner in the fourth-minute and their lead was extended by two Gareth Davies penalties while Liam Williams was in the sin bin for taking out Cuthbert in the air.
Scarlets brought themselves back into the game when George Earle charged down Dan Fish's kick and Jake Ball gathered to send Rhys Priestland over.
Despite going under the posts to make his conversion simple, the Wales fly-half somehow managed to shank his kick onto the post.
Further penalties from Davies and Priestland meant the Blues led 14-8 at half-time.
Both sides were reduced to 14 men ten minutes into the second half as Williams was sent off for a second yellow card offence, this time for deliberately killing the ball at a ruck.
Blues No.8 Robin Copeland followed him down the tunnel swiftly after for dangerous play.
Scarlets brought themselves back into losing bonus point range with seven minutes remaining as they attacked from a five-metre line-out and Ken Owens capped a fine personal display with a try.
Crucially though Steve Shingler, on for Priestland, missed the conversion from wide on the left and the Blues held a one-point lead before Davies' fourth penalty of the afternoon secured a 17-13 win.
The result condemned Scarlets to their fourth straight Welsh derby defeat of the season.
Cardiff Blues celebrate their victory
The Ospreys meanwhile kept up their hunt for a place in the PRO12 play-offs with a 20-10 win over the Dragons.
With the first half finishing scoreless, despite Osprey Ryan Jones spending ten minutes in the bin, a Dragons counter-attack saw Richie Rees touch down against the run of play in the 46th minute.
Dan Biggar brought the Ospreys onto the scoreboard with a penalty after Nic Cudd had been sin-binned for shirt-tugging.
The Ospreys finally broke down the stubborn Dragons defence with 20 minutes remaining as scrum-half Tom Habberfield sniped over from the base of a ruck.
With Jonathan Spratt becoming the second Osprey to see yellow, Tom Prydie was able to level things up at 10-10 for the Dragons.
A Davies' penalty restored the Ospreys' three-point advantage before 18-year-old Tyler Morgan was the fourth player of the afternoon to be sent to the bin four minutes from full time.
Ospreys sealed the win, and denied the Dragons a losing bonus point, with a late penalty try at the scrum.
Despite a defeat and a yellow card, Morgan described making his PRO12 debut in front of 30,000 fans during Judgement Day II as "unbelievable" and there was further thumbs up from Morgan's future Wales teammate Alun Wyn Jones.
"The day was really good," Jones said. "Overall I think the occasion lived up to what it could have been. It's a good product and it's nice to be a part of."
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