It sounds dramatic, and has cultural resonance - I would guess that is the reason why it's phrased in this way, to fit the narrative style of the author, and the world of the novel, as Eric S said. 'He was as black a pirate as any that ever raised a sail' conjures up images for anyone raised in an English speaking country and reading books like treasure island etc. For me it has the sound of a tale told by old sailors in a pub like that featured in treasure island, just saying ' he was really wicked' or something like that just doesn't have the same ring to it. In the phrase used, the author effectively conjures up the whole world of pirates, seafaring, the feel of the imaginary world he is creating and so on.

For me it's a literary choice between a richer use of language or a mundane use of language to describe Euron. Good writing is an art.

English has a lot of ways to say the same thing. In the last century or so, some ways have become more popular than others, and our working vocabulary has become smaller and smaller. If you read historical texts, they tend to use this kind of language - we can understand the words, but they're used in uncommon ways. Since this passage is (I'm guessing) from the fictional world of A Song of Ice and Fire, which is modeled after medieval Europe, the author would tend to use the more uncommon way to say something.