Priestland found his way into many pundits' teams of the tournament in New Zealand and he was at it again as the welsh region put a black mark on Saints boss Jim Mallinder's England credentials.
Mallinder has been heavily tipped to take over with the Red Rose outfit after Martin Johnson's departure this week but it was all about Priestland and co. at Franklin's Gardens.
The Wales No. 10's 13 points came from a try and the conversions from his and three other scores from Liam Williams, Aaron Shingler and Matt Gilbert.
Last year's beaten finalists rallied late on with scores from Tom Wood and George Pisi but it was not enough to stop RaboDirect PRO12 strugglers Scarlets claiming their second victory and top spot in Pool One.
Scarlets coach Nigel Davies said: "We had to play a very good game of rugby to get a result here and that is what we did.
"This is pretty big against a side of Northampton's quality. I don't think they have lost a European encounter at home since 2007 so it is a big scalp for us.
"We have to build the momentum. The big thing has been belief, believing we can come to places like this."
While the welsh region's victory at Northampton was a famous one it was edged out in the performance of the night award by Edinburgh - who turned a 24-point second-half deficit into a 48-47 win over Racing Metro 92.
Greg Laidlaw was the Edinburgh hero, with 23 points while Tim Visser's pair of tries, including the all-important late score helped settle a breathtaking clash.
And on a memorable Friday night for the RaboDirect PRO12's representatives in Europe Cardiff Blues also made it two wins on the spin by beating London Irish 24-18.
Rhys Thomas and Lloyd Williams were the try scorers for the Cardiff outfit in a game which hinged on a red card for Exiles centre Steven Shingler after just 20 minutes.
Four penalties and a conversion from Dan Parks were the Blues' other points while Tom Homer's six penalties kept Irish in touch.
Meanwhile Aironi were on the wrong end of a 54-3 hammering against Clermont Auvergne.
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