This is a past participle phrase acting as an adjective modifying "damage".
Here's why:
* "Done" is the past participle of the verb "to do".
* In this context, it's used to describe the state of the damage (it has already happened).
* Using "was done" would make it a passive voice construction, which is grammatically correct but less concise in this case.
Example:
"The damage done by the storm was extensive."