Domain Expansion

User says ," Domain Expansion." and it generates a field which (inside range) generates a 'guaranteed' d12+5 damage (1 extra damage per level past level 5). Can be broken if someone comes in from the outside or if someone of a higher level cast one of their own. From the outside, it looks like a black dome. From inside, it looks however the user wishes it to. can be used once per long or short rest.