An edible but poisonous potion concealed by illusion magic. It resembles a healing potion or another beneficial draught in appearance, smell and taste; an identify spell reveals the deception.
Use the 5e Potion of Poison rules (2014): drinking it deals 3d6 poison damage, with a DC 13 Constitution save to avoid being poisoned. While poisoned, the drinker takes another 3d6 at the start of each turn and may save again at its end. Each successful save reduces subsequent damage by 1d6; the effect ends at zero.