The Ecruteak Gym is a dark path, leading into trainers. At the end, you'll find Morty, the gym leader, and battle him. When you've won, you'll get the Fog Badge, and TM30 Shadow Ball. All the trainers are Mediums. The Fog Badge lets you use Surf in the field and makes all Pokemon up to Lv.50, even the ones obtained in a trade, obey you.