Externals, or High Lords, are a group of mutants who possess immortality in addition to their natural gifts. The immortality typically kicks in after their first death, "freezing" the External at that age indefinitely. After that, an External can only be killed by "severing the five branches", presumably a reference to removing their head and limbs. Despite the legends, Externals proved vulnerable to the Legacy Virus and to Selene's life-absorbing powers.
The difference between Externals and High Lords was never made clear. Some references made it appear the two terms were interchangeable, while others suggested one referred to the immortal mutants in general, while the other specifically referred to the cabal Gideon and Saul belonged to.