It’s an archery term.
To ‘draw a bead’ on something is to take aim at something with your arrow (and has translated to gun hunting since then in some circles - ‘to draw a bead on your target’).
It is a reference to Mafydd’s fate, and Gwyn’s role in it. If I had written the last chapter, Lludd would have made specific mention of the term. The fact that it’s in past tense (‘the drawn bead’ as opposed ‘to draw a bead’) suggests that the aim has already been taken when you start reading the story.
In other words, they were doomed as soon as they met each other. The arrow was already pointing at the target, so to speak. The archery symbolism would have been more obvious, I think, maybe, who knows, if I’d written that fifth chapter; but the idea was there would be archery in the first chapter (Mafydd and Gwyn going off to shoot arrows together and comparing their skills), and tragically, there would have been archery in the last chapter too.