The creature's past and even name are unknown; it is contained in a stone called the Wraith Stone that is found by Amanda in Paraiso. The full extent of it's powers include throwing fireballs, teleporting and merging with other beings - one who obtains the stone can have complete control of the entity.
After the events of Legend, Lara keeps the Wraith Stone, but it is stolen from her by her doppelganger and given back to Amanda. Amanda later uses it to help Lara fight Thralls in the Arctic Circle level as Lara goes to find her mother.
From the official site
An unexplained malevolent entity of unknown origin, Lara first encountered it in some ruins near Paraíso, Peru where it slaughtered most of Lara's colleagues.