Where does “fagl” come from?

fagl (Middle Welsh) comes from Latin facula, from Latin fax, from Proto-Indo-European ǵʷʰeh₂k- — to shine.

Ancestry of “fagl”, step by step

Step	Language	Word	Meaning
1	Latin	facula	small torch
2	Latin	fax	torch, firebrand; fireball, comet; cause of ruin,...
3	Proto-Indo-European	ǵʷʰeh₂k-	to shine