The Tempest is an Eldar super-heavy grav-tank. Like the Deathstalker, it has not been seen since the days of Epic. The Tempest is armed with an extremely potent double barreled laser cannon which is easily capable of destroying the most heavily armored opponent. Turret mounted shuriken cannon and several shuriken catapults on the hull provide protection against attacking infantry. The sight of a super heavy vehicle on the battlefield is enough to send many troops scurrying for cover.
From what we can gather, this thing will probably rape infantry with the amount of Shuriken-dependent weaponry it carries around and melt vehicles with its main cannons. Unlike other Eldar super-heavies, it doesn't use the same chassis as the Scorpion, but of course this model came out way back in the 1990's. On the other hand, most people like to convert the Tempest into a Scorpion proxy or use the rules directly from the Scorpion, modify it a little bit and play the Tempest like its own vehicle. An explanation on why the Scorpion is always used is due to the similarities in their armament, with some proclaiming that the Scorpion could be a successor to the Tempest of yore.