Shadows as in those cast by living people, not shadows as in blackish incorporeal undead.
Undead meat also meaning zombies, vampires, skeletons and the like--corporeal undead. Incorporeal undead are fair game.
All a wend can see when it encounters something is its shadow, so it will always attack the shadow first. If something gets in the way of its meal (like the person the shadow is attached to), it'll attack it, but shadows to a wend are first targets.
Think I'm gonna pull out that 90 degree thing--just make it so that they can see shadows of living creatures, and I'll add that they attack shadows first.
As to the comment that there are packs of these things--just 2 casts of Light and you're as good as invisible to these guys. Light, being a cantrip, is something EVERYBODY is going to have. Maybe not while traveling overland, but really, what moron doesn't bring more than one torch?