This quote looks like it comes from a type of web novel (specifically fantasy stories of martial arts, righteous warriors, and immortal sages).
「惊鸿」refers to a swan goose「鸿」which flies straight upwards (like a missile/jet) whenever it gets startled「惊」, and harks back to the idiom「翩若惊鸿」, which is an old chengyu describing the graceful body figure and mannerisms of immortal beauties.
In this context, the novel has set the usage of the word「惊鸿」. Either this sword being called「惊鸿剑」can really transform into a「惊鸿」, or「惊鸿」is a metaphor for its powers, or「惊鸿」is just one of its powers. Describing the sword as being able to transform into a lightning bolt gives much the same imagery.
「道」is a measure word, but its use here is innovative.「一道X」gives an impression of a flash of lightning, highlighting the swiftness and nimbleness of a sword's motion. In literature, a measure word (that may be different from expected) may be used to bring out some special features of the object measured for the audience. For example, I could use the word「闪」to measure occurrences of lightning.