Unknown Entity
The Unknown Entity is a monster first encountered in Peru and later in Kazakhstan, brought there by Amanda Evert.
The creature's past and even name are unknown,it is contained in a stone called the wraith amulet that found by Amanda in Paraiso.The full extend of it's powers include throwing fireballs,teleporting and mering with other beings,one who obtains the amulet can have complete control of the entity
In Bolivia - The Looking Glass Amanda will merge with the creature to form the Amanda Entity.
In Underworld
It is unknown if the entity will be returning in Underworld,however in the launch trailer,Amanda is seeing using the wraih amulet to keep a Yeti Thrall at a distance.
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.