Drops off of these dudes. Pretty common blueprint drop at 1.7% per the wiki.
edit: the rarest drops are Spiteful Sword, Bow & Endless Quiver and Bloodthirsty Shield all at 0.03%.
Amzingly, I already have Bloodthirsty Shield.
1.7% may sound like a high rate but these guys only appear in one biome and in fairly small numbers, you fight maybe 5 of them in a run.