Frostfire Lantern

While lit, the flame in this ornate mithril lantern turns blue and sheds a cold, blue dim light in a 30-foot radius. After the lantern's flame has burned for 1 hour, it can't be lit again until the next dawn. You can extinguish the lantern's flame early for use at a later time. Deduct the time it burned in increments of 1 minute from the lantern's total burn time. When a creature enters the lantern's light for the first time on a turn or starts its turn there, the creature must succeed on a DC 17 Constitution saving throw or be vulnerable to cold damage until the start of its next turn. When you light the lantern, choose up to four creatures you can see within 30 feet of you, which can include yourself. The chosen creatures are immune to this effect of the lantern's light.