Scepter

The Chitauri Scepter, also known as Loki's Scepter or Loki's Staff, is a scepter that utilizes a blue gem powered by the Tesseract. It was wielded by Loki Laufeyson in the Invasion on Earth. It was gifted to Loki by Thanos.

The Avengers
The scepter had a long golden handle and a black fitting which held a blue gem encircled with silver blades. It allowed its owner to open rifts through dimensions with the Tesseract as a gate key, fire powerfully destructive blue blasts of energy and bend the wills of others by controlling their hearts.

Loki could transform it at will from its long, fully-extended appearance to its smaller but equally deadly form the same way he transformed his own appearance with and without his full body armor and cape. He also disguised it as a walking cane when disguising himself amongst humans.

The staff also functioned as a spear, being able to violently impale and slash enemies, besting most in combat with ease. It was even strong enough to match Thor's mighty hammer Mjølnir in close-quarters combat.

The Scepter was also what closed the portal between Earth and the Chitauri's Mother Ship; when Loki took over Erik Selvig's mind, Selvig was still part conscious when his mind was taken over, so he placed a default in the portal opening, If the scepter would hit the Tesseract the portal would close. So once the fight between Loki and The Avengers from the Stark Tower had finish, Black Widow was able to retrieve the Scepter and break the portal connection.

Trivia

 * Due to the appearence of Thanos, fans have speculated that the blue crystal on the secepter is the Mind crystal from the Infinity Gauntlet.