Icarus was a dangerous street drug that developed in Gotham City during the Zero Year. Although it was initially put a stop to by Barry Allen, it resurfaced years later. Icarus had the trait of making it's user spontaneously combust.
Originally sold in Gotham City during the Zero Year, the seller was the corrupt Detective Spencer. After Spencer was killed by Harvey Bullock, the drug disappeared. It resurfaced many years later, being sold by Holter and the Kings of the Sun who had harnessed a metahuman to make batches of the drug.
Much like other street drugs, Icarus gave users a very good and uncontrollable high. However, it also caused their body temperatures to rise to great amounts, eventually resulting in them spontaneously combusting.