The Dungeon Master’s Guide recommends that an adventuring party be comprised of five adventurers with all four roles being covered. Of course there are going to be instances where only three of four roles are represented in an adventuring party. Maybe you’re playing in a public game, like and LFR adventure, and none of the players brought a PC to fill one of the roles. Or maybe none of the players in your group enjoy playing a particular role.
Some would argue that the controller is not an essential role. Of the four roles most players feel the controller is easily the most dispensable. About a year ago we ran a poll asking which role a party could do without. The controller won that dubious honour hands down. While the early heroic tier might support this viewpoint, it quickly becomes evident that controllers are indispensable by the time a party hits the middle of the paragon tier. Controllers are more than capable of carrying a party forward to victory. The fact is you want a controller in your party.
But what happens when this vital member of any adventuring party is missing?