Thunder Dome

Create a 20ft radius dome of energy around yourself. If there are no hostile creatures within 20ft of you, the spell fails. Attacks made against creatures within the dome by creatures outside the dome are done with a -8 penalty. When a creature within the dome deals damage to another creature within the dome, add half of the damage dealt to the dome's counter. If the damage dealt is thunder damage add the full amount.

A creature attempting to willingly leave the dome must make a charisma save against the caster's spell save modifier or be forced to end their turn. If a creature leaves the dome for any reason, they receive thunder damage equal to the dome's counter. If the dome's counter is less than 20, they instead take 20 thunder damage.

A creature that has left the dome may not re-enter the dome. If all conscious creatures within the dome are not hostile to each other it disappears. When the dome disappears in this way all remaining conscious creatures gains temporary hit points that last 1 minute equal to half the dome's counter divided by the number of recipients.