The Shadow is the Bigger Bad of Amnesia: The Dark Descent. It's known as the mysterious guardian of the orbs which appears in the game as an unseen force chasing Daniel relentlessly. Wherever it passes, it destroys anything on its path and leaves a organic fleshy residue behind it which is harmful to touch.
Shadow is known to appear in certain areas of Brennenburg Castle and makes it appearence everytime Daniel makes a progress in that certain area. Sometimes it chases Daniel directly, forcing him to run. It's also the one that blocks the huge gate in the enterance hall which leads outside of the castle.
In the end, depending on player's actions, it will either kill only Daniel, kill only Alexander or kill both of them. It's also revealed in the end that its only intention was having the orb back which was accidentally stolen by Daniel during the archeological expedition in Algeria as if Daniel destroys the portal, rendering the orb to be unused, it will kill Alexander and spare Daniel, who will then leave the castle without looking at his back.