A RESAT is a machine designed by the Elgen based on the powers of Nichelle, without her weaknesses. The name is believed to be derived from the word 'TASER', spelled backwards.
We first see a RESAT in book two: Rise of the Elgen. The Glows of Michael's group, the Electroclan are attacked near the beginning of the book and are placed under the power of the RESAT. The RESAT has the power to absorb the Glows electricity, thus sucking most of the life out of the victim. In Storm of Lightning Hatch has a RESAT 2.0 a.k.a RAVE. This does not need RESAT darts and is a handheld device.