Barn Advisor

The Barn Advisor is one of the main antagonists in the 2007 Valve Game Half-Life 2: Episode 2 and is commonly speculated to be the hidden main antagonist of both the game and series as a whole.

Half-Life 2: Episode 2
The Barn Advisor ejected itself to safety from the Citadel during the events of Episode 1 and crashed in a nearby barn, emerging from its pod and building protective life support structure around itself to rest and mature in growth and abilities. Gordon and Alyx encounter the crashed pod and begin experiencing powerful illusions and telekinetic disturbances as they get nearer to the Advisor. Alyx and Gordon decide to kill it before it wakes up by shutting off its life support system.

Gordon does so using the Gravity Gun, however all this does is cause the structure to peel back and reveal the now-awakened Barn Advisor, who casually slams Gordon and Alyx up against the wall and begins testing its powers on the surroundings, picking up objects and draining a dead man of his blood before twisting him and effortlessly throwing him across the room. It then decides to feed on Gordon until the structure encasing it explodes, resulting in it being injured and enraged by a bar being stuck partially in the side of its head. Deciding to leave immediately and heal itself, it tears the roof to shreds and tosses Gordon and Alyx to the ground, using the pieces of the roof as a powerful makeshift shield swirling around it before flying away.

Trivia

 * It is a popular theory that the Barn Advisor is in fact the main antagonist of the entire series, being the one giving orders to Breen in Half-Life 2 as well as the one seen escaping in Episode 1 after telekinetically attacking Alyx and Gordon and giving orders to Combien squads. It is also theorized that the Barn Advisor is the one who attacked the Silo as well as one of the two Advisors who participated in Eli's death.
 * It is also a popular theory that the Barn Advisor is Wallace Breen, having possibly survived the Citadel Explosion and his mind planted into a host body in order to truly become one of the leaders of the Combine. This may be supported by the fact that the Advisor, upon awakening, tests its powers on its nearby surrounding in an almost childlike curiosity as though having not used them before.