Eitr

Eitr is the fluid of life in Nordic mythology, and is the origin of all living things. The first giant Ymir was conceived from eitr. The substance is supposed to be very poisonous and is produced by Jörmungandr (the Midgard Serpent) and other serpents. The word eitr exists in most North Germanic languages (all derived from the Old Norse language) in Icelandic eitur, in Danish edder, in Swedish etter. Cognates also exist in Dutch ether, in German Eiter (lit. pus), in Old Saxon ĕttar, in Old English ăttor. The meaning of the word is very broad: poisonous, evil, bad, angry, sinister, etc., and according to Wikipedia the word is used in common Scandinavian folklore as a synonym for snake poison [1].

From Teutonic Mythology by Viktor Rydberg [2];

Eitr means the lowest degree of cold and poison at the same time ... tongues were made speechless with cold. In Saxo’s descriptions of the regions of misery in the lower world, it is only the torturing demons that speak. The dead are speechless, and suffer their agonies without uttering a sound; but, when the spirits of torture so desire, and force and egg them on, they can produce a howl.

Tomb Raider Underworld

The Thralls are born from this fluid. It has a bright blue color, and is found in Helheim. During the climax of Underworld, Amelia Croft, the Doppelganger and Natla all take an unwanted bath in Eitr.
If Eitr creates Thralls, then presumably Amelia - who already is a Thrall - would have been unharmed by the experience.