(With all apologies to the Finns, as the Sammas is another name for the Sampo.)
According to the legends, the Sammas is a large, ornate, ivory drinking horn, carved with magical runes, which is always full of water. The legends are exceedingly hazy on its pedigree, although must suggest it was made by an unnamed craftsman to purify the River of Tears, grown corrupted by the blood of dead primordials, or that it was made to provide enough water for the craftsman's village. (A few legends claim the Sorrowfell Plains used to be desert, and the Sammas is the vessel that made them the verdant plains they are today. These tales also tend to claim that the dead god Sorg made the vessel.)
The important, common element is that a craftsman needed a lot of pure water, and so made the artifact.
The Sammas is similar to a decanter of endless water, in that it produces limitless water. However, there is no variance to the amount of water — if poured out, it always pours at a rate of one gallon every six to ten seconds or so. (One gallon per round, like the "stream" setting of the decanter.) Unlike a decanter, though, this water is pure (and should likely be treated as holy water). So pure, in fact, that it purifies things it touches. Water from the Sammas will decontaminate food, drink, people, and objects as per the spells purify food and drink, neutralize poison, or remove disease (or their local equivalent in whatever game system you happen to be running). It will also purify areas and people of various types of corruption, and possibly even throw off haunting or possessing spirits.
The main problem with the Sammas is its location. It has been lost for centuries, and no one has yet determined its location. It is commonly thought the be hidden somewhere in the Hoarfrost Ridge, far to the north, but it could legitimately be anywhere. (It is thought that some shrines advertise a connection to the Sammas either as an attraction to bring pilgrims or as a trap for the unwary.)
The last person to seek the Sammas was the fey noble, Lady Graunwen, who departed with a retinue of elves to seek the artifact in the autumn. Since her final message last winter, two search parties have gone to find her. Neither has returned.