Wales prop Maisie Davies and fly half Carys Hughes scored all visitors points against their Welsh rivals in Llanelli.
Davies scored a late try while Hughes kicked a penalty and conversion to keep her 100 percent success rate from the tee in the Celtic Challenge alive this season.
Brython won the first game 30-5 at the home of the Scarlets last season but this was a very different contest with Gwalia recording a first historic away win against their rivals.
More than 600 Welsh rugby supporters turned up to witness a typically intense and physical confrontation between two sides that are building a real rivalry and healthy respect for each other on the field.
Wales scrum half Ffion Lewis returned to action after nearly two years to play her first game on Welsh soil for Brython.
Gwalia won the first Welsh derby over the festive period with a 42-22 win against Brython before Christmas at Ystrad Mynach.
Both Welsh sides now hit the road with Brython travelling away to face Edinburgh in Scotland and Gwalia playing Clover in Ireland on January 4th.
In a nervy and tense opening, both sides struggled to find any fluency until a concerted forward drive saw Brython concede a penalty and Hughes added the three points to give Gwalia an early lead.
Thunder scrum half Seren Singleton then delivered a bone-crunching tackle to rally her side as they were under pressure from the Lightning pick and go driving game.
Brython then enjoyed their first real period of pressure of the game after a break by wing Eleanor Hing and a deft chip by fellow wing Ellie Tromans saw the home side camped on the visitor’s line.
Gwalia defence held firm for a second time when Thunder hooker Evie Gill was held up over the line and despite the home side’s intense pressure toward the end of the half, Gwalia went in 3-0 ahead at half-time.
After the break, both sides probed at each other with wing Tromans for Brython and wing Catherine Richards for Gwalia showing some nice attacking touches.
Wales centre Meg Webb and Brython replacement fly-half Hanna Marshall both made half-breaks, but it was Gwalia prop Davies who barged over from short-range to make it 8-0.
Fly half Hughes converted to land her eight successive conversion in the Celtic Challenge as Gwalia stormed to the top of the points table after two rounds of action.
