The material which would be later known as Isotope-8 is an item used in Marvel: Avengers Alliance and was brought to Earth by a mysterious Pulse, a phenomenon which also affected technology worldwide and left strange energy signatures.
The international security organisation S.H.I.E.L.D. noticed this Pulse, but they were not the only people to do so. Rival organisations Hydra and A.I.M. also tried to obtain the strange material left behind, and battled with S.H.I.E.L.D. for control of it. With the help of an alliance of superheroes led by an Agent, the two terrorist groups were drawn back.
After fending off the rival organisations and researching the "mysterious ore," S.H.I.E.L.D. arrived to the conclusion that the Pulse was a charged cloud of an "Isotope-8," a material was capable of multiplying people's powers. The Alliance's new task would become that of securing the Iso-8 from supervillains, some of whom would even create their own alliance, a syndicate led by Doctor Doom.
Both S.H.I.E.L.D. and the Syndicate used the Isotope-8 to create different types of armaments and to empower their forces.
Different types of unstable Isotope-8 later appeared, including a green Iso-8; a blue Iso-8 mutated by the Brotherhood of Mutants; a cosmic yellow Iso-8 created when the Phoenix Force interfered in the Pulse's path towards Earth; a red demonic Iso-8 spawned from Hell; an Asgardian purple Iso-8 modified by its contact with the magic of Asgard; a derivated purple Iso-8 developed by Hank Pym; a "chaotic" Iso-8 created by Ultron; a red Iso-8 created from the demonic Iso-8 synthesized by the power of Tony Stark's arc reactor, and a violet Iso-8 variant created by Wonder Man.
The true nature of the Isotope-8 is unknown. Mutable at a subatomic level, the Iso-8 attaches itself to the essential characteristic of a material it touches and intensifies it. According to Otto Octavius, the Isotope-8 can be considered "the perfect catalyst," although Reed Richards has stated it is not because, unlike catalysts, the Isotope-8 also changes itself.
If the Isotope-8 is put in a crystal array to focus a laser, it makes the laser have more focus. If it's put in an electrical field, it magnifies the charge and the momentum of electrons, and becomes magnetic. If it is implemented as part of a plasma, the plasma gets denser, hotter and more powerful. Mr. Fantastic has studied the reactions of Isotope-8 with heat, electricity and magnetism, and stated that in those areas, the Iso-8 is an "energy intensifier the likes of which I've never seen."
Hydra has managed to condense data down to binary code and transmit it to limbs and muscles via Iso-8, allowing themselves to create almost perfect replicas of the Avengers from footage of different heroes in action.
Alternate Reality Versions
Iso-8 is used as a power-enhancing material in many of Marvel's mobile games. There is also a version in the comics that is more commonly known as neutronium.
|On Earth-12131 Iso-8 comes in a variety of colors and crystalline structures. These crystals are used to empower and strengthen characters.||Marvel: Avengers Alliance
(March 1, 2012)
|On an unidentified Earth Iso-8 is used for a temporary power boost during battle.||Marvel War of Heroes
(19 October 2012)
|On Earth-199999 blue Iso-8 crystals were used to speed up the time required to upgrade Iron Man's armor.
Thor and his allies used purple Iso-8 crystals to upgrade their abilities.
Captain America used a variety of Iso-8 crystals to upgrade his strength and abilities.
|Iron Man 3: The Official Game
(April 25, 2013)
|On Earth-TRN665, purple Iso-8 crystals are used to increase the power of team members, and red Iso-8 crystals are used to increase the strength of equipment.||Marvel Puzzle Quest
(October 3, 2013)
|On Earth-616, the exo-space element known as neutronium forms a stable isometric 8-atom arrangement that can grant great power to the possessor.||Mighty Avengers (Vol. 2) #9
(April 9, 2014)
|On Earth-TRN842 a variety of Iso-8 crystals could provide bonuses to offensive and defensive skills.||Guardians of the Galaxy: The Universal Weapon
(Jul 17, 2014)
|On Earth-TRN461 purple Iso-8 crystals provide power to operate the portals that summon characters from other dimensions.||Spider-Man Unlimited
(September 10, 2014)
|On Earth-TRN517 Iso-8 in a variety of colors is used to empower the assorted champions.||Marvel Contest of Champions
(December 10, 2014)
|Never pictured||On Earth-12041 Hawkeye defends an unseen cache of Iso-8 from an invading Hydra force.||Avengers Assemble: Tower Rush
|On Earth-TRN012 Iso-8 crystals in a variety of colors are used to provide fighting bonuses to team members.||Marvel Future Fight
(April 30, 2015)
|On Earth-TRN765 Iso-8 is used to upgrade and customize a character's fighting ability.||Marvel Ultimate Alliance 3: The Black Order
(July 19, 2019)
|On Earth-TRN670 green Iso-8 crystals are used to give characters ability bonuses.||Marvel Strike Force
(September 29, 2020)
- Iso-8 was developed as an in-game resource for Marvel: Avengers Alliance (March 1, 2012) over two years before it was introduced in the comics in Mighty Avengers (Vol. 2) #9 (April 9, 2014) as neutronium. The comics element would not receive the name Iso-8 until Ultimates (Vol. 3) #1 (November 11, 2015.)
- According to Marvel vice president of business development Sean Haran the inclusion of Iso-8 in multiple mobile games was an "attempt to connect — even if it’s a loose thread — the games that we’re creating day-to-day." 
- Writer Al Ewing worked with Marvel Games Creative Director Bill Rosemann on how to introduce Iso-8 to the world of the comics.