Rottie The Rotworm is the boss of the rotworms in Rookgaard. It spawns randomly (typically only 1 per day)
Rottie The Rotworm spawn inside the Legion Helmet quest or the Ghoul Quest in Rookgaard:
Treat it like a stronger version of a Diabolic Rotworm, at level 20+ you should be able to kill Rottie without much problems.
He is not really fast so you can easily run through the room while shooting arrows. You can also consider using the ghoul teleport to run through the beginning of the room and doing that like in circles.
Rottie The Rotworm loot may vary from some platinum coins, gold ingot, etc.