http://www.maplewiki.net/index.php?title=Sprite_Summon
There we go. Those are the base damages. I believe the sprites attack every few seconds and up to 6 monsters at a time. The level affects the range of the sprite.
Eh, I haven't got a Knight of Cygnus to end game so I dunno. However, I can say that I haven't yet met a NW with maxed darkness, but that might just be me. Generally the other 1st job skills seem to be more needed?