There would be two versions of this mob.
First: The normal Ghost. It would be a transparent testificate that would flee when you look at him. When he does sucseed in attacking you, however, he causes the Nausea effect and does minimal damage. Maybe 2 1/2 hearts.
He would spawn in strongholds.
Second: The Ender Ghost. This would be a completely new model. Transparent.
He becomes aggro when you attack the Endragon. He flies around the obsidian towers otherwise (if that is possible). He does a lot more damage than the normal ghost, maybe 7 with no armor. Not sure if he should cause an effect or not.
Please comment if you like them or would like to suggest changes.