The Stranger

The Stranger is the overall main antagonist of the Escape from Furnace book series.

Description
The Stranger is an ancient being that is described as being composed purely of evil. It is purely black, with fog rolling off of it. It can speak a variety of languages, with it's head "folding like origami" when it speaks.

Execution- Book 5
The source of the Nectar and the more powerful, highly concentrated stuff holding Alfred Furnace together, The Stranger is a mystery monster that simply wants to end the human world and rule the mutated monsters left standing. SPOILER --- Alfred Furnace's last act of defiance against the Stranger was to get Alex to take control of Furnace's armies and allow them to peacefully laid to rest forever instead of ending the world. The Stranger is killed when Colonel Panettierre transfers the Stranger into her and out of Alex in a desperate bid for power, but because she's an adult, the Stranger cannot live inside her and they both explode.

Nothing is known about the Stranger's origins or what it was before it met Alfred Furnace. At one point in the book the question is asked if there are more like the Stranger, and chillingly it goes unanswered.