The Shadow is one of the bosses in Wayward Souls, he is the final boss of the game and is encountered on Floor 20 of the Diamond Dungeon, the Labyrinth. The Shadow is part of a larger group of beings like him, that lured you into the Labyrinth, and tempts you with an offer to join them. The offer differs between classes. The Shadow appears on floor 20 of the Labyrinth.
The Shadow only has one attack, which is small bolts of poison falling from the sky and doing minor area damage. He summons medium sized groups of enemies from any of the other areas. (Mines, tower, drudges, etc.) Once you have defeated all of the enemies, The Shadow appears, and you have a short amount of time to hit him before he summons more enemies and fades away. He repeats this process, occasionally attacking you while you fight the other enemies, until one of you is dead.
All in all, The Shadow is one of the easier bosses in the game. The groups of enemies are fairly small, and all you have to do to avoid his attacks is not stand in the same place for too long. You should only need to defeat three or four waves of enemies before you can deal enough damage to him. Scrolls of fireball for fighting the clusters of enemies, and poison potions for the boss will quickly win you the battle, but any character that can make it to him should have no problem in defeating him.