"The Externals" is the name several mutants gave themselves when forming a loose-knit group of mutants who believed themselves to be immortal. The criteria for being an External remains indistinct, but in general all Externals appear to be of a sub-species that are able to recover from mortal wounds and even death itself. However, over time, several mutants thought to be Externals where deemed not, and perhaps the distinction is a capricious one.
The Externals apparently know of each others' existences on a telepathic level, although this telepathy remains vague and perhaps only manifests when an External first resurrects him or herself. It has also yet to be revealed how the Externals originally banded together. Nevertheless, by the time the Externals began making their activities public, their number included eight members: Absalom, Burke, Candra, Crule, Gideon, Nicodemus, Saul, and Selene. (At one point, it was suggested the mutant Apocalypse was an External as well, but this was never upheld by any of the actual Externals.)
In more recent years, the Externals were searching for their next member. Gideon long believed the next External was the hero Sunspot, and had positioned himself in Sunspot's life even before his heroic career. Later, when Sunspot's team, the New Mutants, was at a point of collapse, Gideon orchestrated the death of Sunspot's father in order to allow Sunspot to leave the team and join him. Unaware of Gideon's true purpose and actions, Sunspot agreed.
At the same time, the time-travelling mutant Cable had taken leadership of the New Mutants (renamed X-Force), as he believed, based on his travels in time, that the mutant Cannonball would be an External. When Cannonball was killed by the villain Sauron and later revived, the Externals sensed Cannonball to be an External as well. Gideon sent Crule to kill Cable so that the Externals could easily capture Cannonball, but Crule was defeated. X-Force then confronted Gideon, revealing the truth and freeing Sunspot. Cannonball was made to promise not to interfere with Externals' affairs, although, later, Saul kidnapped three of X-Force's members in order to blackmail Cannonball into joining the Externals. X-Force nevertheless defeated Saul and Gideon.
Months later, several Externals peaceably sought Cannonball to join them again. They hoped that Cannonball might hold a key to solving the Legacy Virus that killed their members Nicodemus and Burke, but Cannonball could not help them.
Later still, Gideon sent a distress call to X-Force after being betrayed by the External Selene. For reasons of her own, Selene was suddenly killing the remaining members of the Externals, absorbing their life forces. Despite the efforts of X-Force, Selene escaped but not before summarily dismissing the assumption that Cannonball was an External, saying it was a ploy by Cable to win the mutant's trust. While with the X-Men, Beast believed Cannonball to be an External.
Candra was seemingly killed by the X-Men, but later resurfaced having survived the destruction of her heart gem as a disembodied consciousness. Although she had not yet fully reconstituted herself, and was re-killed before she succeeded.
Selene, who was the only known verified External still alive, died during her ascension, killed by X-Force on Necrosha. But later was resurrected by Enchantress.