From "Treasury of the Macabre"

This sharpened bone writing implement can be used by a warlock to enhance the power of certain magical writings by infusing your blood into the ink. If used to scribe a scroll of a patron spell, the scroll’s save DC is increased by 1. A glyph of warding, symbol, or similar magical rune inscribed with a witchblood stylus requires those triggering it to make any saving throw with disadvantage or suffer the effects of the glyph or symbol. A witchblood stylus may be used once per day, and using it inflicts 1d3 points of necrotic damage to you per level of the spell being inscribed.

Notes: warlock