Final answer:
King Arthur overcomes the challenge of a terrible dragon and breaks the enchantment of a brazen shield in Arthurian legend.
Step-by-step explanation:
The obstacle that King Arthur overcomes in the provided excerpt is a fierce combat against a terrible dragon. Successfully defeating the creature, he then faces the enchanted brazen shield. His valor and steadfastness lead the enchantment of the shield to be broken, as evidenced by it falling at his feet with a 'mighty great and terrible ringing sound.' This feat reflects the common motif in Arthurian legend of overcoming supernatural or magical challenges.