Dark release is an advanced element from Kirigakure that is a mixture of fire and lightning that typically shows itself as a dark and strong energy. Its pretty unique in that its users receive a special mark (two overlapping diamonds) that appear on the palm of one of their hands. This mark is used to absorb chakra into something of a separate and limitless space where the chakra can be broken down and stored for fueling dark release techniques. Most of the dark release techniques require some amount of absorption to have occurred before they can be used. The absorption/released forms of meiton carry similar, but slightly different strengths and weaknesses. Since absorption works on chakra, its best used on raw chakra and energy based chakras, e.g. fire and lightning, but its weak against solid elements like earth and water. The released form is also weak to earth and water which can withstand it, and is strong against fire and lightning, but its also strong against some less durable solid elements like sand and snow.
Its best known for being used by Hiruko, though I'm not sure if it was an original ability of his or one of the abilities he gained with the chimera technique.