Balaak was a Zygon military commander.
After their homeworld Zygor was destroyed, he and a small group of Zygons crashed on Earth around the 17th century and were stranded. He wanted to turn Earth into their new homeworld, planning on using a horde of Skarasen to destroy humanity to do so. He was thwarted by the Eighth Doctor and killed by George Litefoot. (PROSE: The Bodysnatchers)