Play as an agro MOb (Mobile Object, e.g. monster).
You log in to a special interface, with no choice as to specific MOb or area and get placed with the MOb as your avatar along with all it's abilities and limitations, specifically you can't leave the spawn area. Player MObs (PMObs) are not distinguishable from regular MObs by any outward sign. Ideally you could control some MOb animations, using them like emotes when you would rather be known. Possibly, once the fight is engaged your PMOb name appears above your head, after all, it would be nice to get a reputation.
All player names and titles are not visible to PMObs. PMObs have their own chat channel and cannot access any player channel. Players are only rendered when inside the normal agro range for that MOb (or maybe a bit larger, it depends on what turns out to work best for balance). Any player that attacks the PMOb or heals a player becomes visible to the PMOb. Taunt doesn't force you to attack the taunting player, only, if successful, makes him/her the only player that is rendered in your view.
You start as a weanie ragus (or similar) and for every player kill you get you gain mob skill points. After a certain number of points you level up and can choose to play a harder mob. For every death where you don't inflict any damage you lose mob skill points. Only MObs that are normally agro can become PMObs. When killed PMObs give a bit more experience than regular MObs.
When you die you don't lose any skill points (it is your reason d'etre after all) you immediately possess another MOb, no control of specific MOb or area. You can choose to abandon your assigned mob at any time when you are not fighting, after all, if the players are leaving there is no point.
If you are inactive for x amount of time the MOb regains it's AI though you can still see from it's perspective. The value x is on the order of a few seconds when engaged in combat.
PMObs are assigned to spawns by whatever game mechanic can be leveraged to try to hook up PMObs with players.
Air-castle stuff I know, but you asked!