Tesseract

The Tesseract is an Infinity Stone and an Asgardian relic of ancient unlimited power, and was considered as "The Jewel of Odin's Treasure Room" by Johann Schmidt.

History
The Tesseract is an ancient Asgardian artifact of unimaginable power. It was kept in Odin's artifact chamber but came to Earth by accident. It was hidden in a Viking church in Norway and kept under the Tree of Life. In World War II, Johann Schmidt invaded the church and stole the Tesseract. He used the Tesseract to build extremely powerful weapons that disintegrated humans on touch (the weapons also severely damaged normal objects). Johann also used the Tesseract to make large bombs that could destroy capital cities. Johann held the Tesseract before he sent the missiles and it disintegrated (or maybe teleported to another dimension) him on touch. The cube fell in to the Arctic ocean and was found by S.H.I.E.L.D.'s creator Howard Stark some time later. The Tesseract was kept under guard by S.H.I.E.L.D. to make sure that it didn't fall in to the wrong hands. In 2011 Nick Fury called in Erik Selvig and asked him whether he would help S.H.I.E.L.D. with making the Tesseract an unlimited power source.

The Avengers
After falling into the abyss of space, Loki eventually comes into contact with the alien race of the Chitauri and their masters, Thanos and his servant "The Other". Loki promises to retrieve the Tesseract from Earth and give it to them in exchange for their army.

Around this time, S.H.I.E.L.D. is planning to use the Tesseract to create weapons of mass destruction so Earth can defend itself in the event of an attack by enemies from other worlds. This was due to the battle between Thor and Loki, however, according to Thor, their work with the tesseract was what drew Loki and the Chitauri to it.

About a year after Fury has recruited Dr. Selvig to study the Tesseract, he has now been notified that it is "misbehaving". Fury arrives at a Project P.E.G.A.S.U.S. where the Tesseract opens a portal into space and transports Loki to the S.H.I.E.L.D. base. Loki immediately begins slaughtering S.H.I.E.L.D. agents and reveals that he plans to use the Tesseract to enslave and rule over the human race. The power from the portal even creates an explosion that levels the entire nearby area.

After escaping, Loki has the mind-controlled Dr. Selvig begin to work on the Tesseract to build a device.

S.H.I.E.L.D. recruits Bruce Banner along with Tony Stark to trace the Tesseract as it is emitting small amounts of gamma radiation that only he would be able to trace.

Selvig and Loki later set up the Tesseract-powered device on top of Stark Tower and activate another portal which opens the door to the Chitauri fleet. Later Natasha Romanoff is told by Dr. Selvig, now free from Loki's mind control, that Loki's staff is able to shut off the portal.

After closing the portal, the Tesseract is taken back to Asgard along with Loki. Though his superiors are angry at this turn of events, Nick Fury defends his decision to allow Thor to take the Tesseract, saying that humanity is simply not at a level where we deserve the cube's power.

Thor: The Dark World
After recovering the Aether, the Asgardians took it to The Collector and told him the Tesseract was in Asgard and it is "unwise to keep two Infinity Stones in one place." Unknown to the Asgardians, however, The Collector planned to steal the Tesseract alongside four other infinity stones.

Guardians of the Galaxy
The Tesseract appeared on a video footage as The Collector was giving Star-Lord, Gamora, Rocket Raccoon, and Groot the history of the Infinity Stones.

Avengers: Age of Ultron
The Tesseract appears in Thor's vision of the Infinity Stones and their connection in the previous years.

Thor: Ragnarok
The Tesseract appears still in Odin's vault in Asgard. Most likely, Thanos will take advantage of the fact that Surtur destroyed the Vault to collect the Tesseract.

Powers and Abilities
The Cosmic Cube, better known as the Tesseract, is one of the most powerful artifacts in the universe. According to Fury, it holds great power that is almost unlimited. Black Widow states that it has enough power to destroy the entire planet. The behavior of the Cube is inconsistent and random; the Red Skull was apparently incinerated (or possibly teleported) upon touching it.

The Tesseract has the capability to open rifts through space and time. This is how Loki transported himself and later, the Chitauri, to Earth through a portal. Thor and a muzzled Loki use the Tesseract to teleport back to Asgard.

It cannot be held by mortal hands. Any mortal who does so will be disintegrated and have their remains sent to another world, as witnessed with the Red Skull. According to The Other, Loki (and the other Asgardians) know its workings as humans never will.

The Tesseract is one of the six Infinity Stones, the one representing space.

Appearances

 * Thor
 * Captain America: The First Avenger
 * The Avengers
 * Agents of S.H.I.E.L.D. (mentioned)
 * Thor: The Dark World (mentioned)
 * Guardians of the Galaxy (hologram)
 * Avengers Assemble
 * Avengers: Age of Ultron (in a vision)
 * Thor: Ragnarok 
 * Avengers: Infinity War 

Trivia

 * It is based on the Cosmic Cube from the Marvel Comics continuity. The origin of the comic book version is largely unknown, while the cube from the Marvel Cinematic Universe is believed to be of Asgardian origin.
 * A replica of Tesseract is a pre-order bonus for the Phase One - Avengers Assembled Blu-ray. When placed in the S.H.I.E.L.D. briefcase, it glows.
 * A tesseract, commonly called a hypercube, is a 4-dimensional cube. The Tesseract shows some possibly "4th dimensional" qualities, such as: teleporting objects, seemingly through a fourth dimension; being able to call up massive amounts of cosmic energy, seemingly out of another dimension; Howard Stark draws the Hypercube as a four dimensional cube, as a cube within and connected to a larger cube
 * The Tesseract is technically the first Infinity Stone to appear in the movie franchise, the second being Aether (a.k.a. the Reality Stone).
 * It is so far the Infinity Stone most viewed.