The Otherwhere Chalk is a tool added by Witchery. It is used to draw 65 otherwhere, or purple, glyphs used in circle magic before breaking. Rituals that require otherwhere glyphs will usually involve the End and transposition.
If you are standing too close to the cauldron when brewing completes, you will get Slowness II for 45 seconds.
Recipe as of 0.21.0: