David Zangouras, Jason Harris and a penalty try put the Scarlets in front in the second half with Daniel Etteridge adding a conversion, two penalties and a drop goal.
However, Williams stepped up to earn the Blues a point in an outstanding contest. WRU regional performance manager Gafyn Cooper said, "It was an outstanding game and I don't think either side deserved to lose. It was another great advert for the competition."
Deacon and Scarlets replacement Scott Thomas were given yellow cards but that did little to spoil the contest.
Meanwhile, having lifted the under 18 trophy last season, the Ospreys carried on from where they left off last season with a 27-6 victory over the Dragons at Abertillery Park.
Despite having to travel for the opening game of this season, the Osprys ended up on top thanks to 12 points from the boot of last season's under 16 captain Daniel Biggar.
Wing Ryan Bayliss, centre Ashley Beck and the excellent back row forward Rory Pitman helped themselves to tries as the Ospreys took the points at the expense of a lethargic Dragons side.
Dragins outside half Danny Ellis bagged two penalties, but the hosts never recovered from a poor start and gave themselves too much to do in the second period.