A Tosk had the honour of being the first Gamma Quadrant inhabitant to travel through the wormhole into Bajoran space. The individual arrived in a damaged vessel, claiming that it had been incapacitated while passing through the wormhole. In fact it soon became clear that the ship had been deliberately attacked.
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Tosk are Humanoid, but with green skin and bony ridges across their head and neck. They are remarkably self sufficient; a Tosk requires only 17 minutes of sleep per day, and has no concept of rest or recreation. Their bodies have plastic fibres distributed without them which store liquid nutrients, allowing them to go for long periods with little or no food. The individual on Deep Space Nine claimed to have no vices of any kind. Perhaps most curiously, the Tosk have a natural ability to cloak or shroud themselves, becoming invisible to the naked eye.
Tosk continued to refuse to disclose any facts about his reasons for being on the station, and shortly after arriving was caught trying to access a security console and imprisoned by the stations security chief Odo. When the Hunters arrived they revealed that the Tosk had been artificially created to serve as the prey in their hunts. The Tosk had been raised with a strong honour code which valued a "good" death above all else - to be taken captive alive was one of the ultimate crimes that any Tosk could commit, resulting in a life of imprisonment and humiliation. The Tosk code also forbade them from revealing the existence of the hunt to any outsiders, although they were allowed to enlist the aid of others if they could do so without revealing the reason why it was required.
With the assistance of Chief O'Brien Tosk was able to escape captivity and flee the station, leading to a resumption of the hunt. Before leaving the Hunters promised that the wormhole would be placed out of bounds for any future hunt so as to avoid further contact between themselves and the Federation. 
The artificial nature of the Tosk combined with their ability to cloak and almost complete lack of need for food and sleep has led some to speculate that the Tosk served as the basis for the Dominion's genetically engineered Jem'Hadar soldiers. It is not known if this is true, and with the end of the Dominion war the question has become largely academic.