Befoul Concoction

You touch one small vessel, such as a mug, pitcher, or vial, containing a liquid of any kind and contaminate it with a dangerous, and magically poisonous serum. If someone drinks the serum or the mixture that the spell makes, they must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage, and become Poisoned on a failed save, or half as much damage on a successful one.

The serum itself is colourless, and tasteless, but has a subtle, yet defined odor when mixed with another liquid, which requires a DC 15 Investigation check to discover. The serum is also innately magical and thus becomes visible under the effects of Detect Magic.