All artifacts are unique, but there are four objects which the game refers to as unique items (on the discoveries page):

Artifacts are not unique items, as they are a normal type of item with a special name and additional benefits. For example, the Eye of the Aethiopica is a named amulet of ESP that also provides faster energy regeneration amongst other extra powers.

Stormbringer and the Tsurugi of Muramasa are the only runesword and tsurugi, respectively, that can normally be generated in the game, but neither count as unique items. It is, however, possible to find a non-artifact runesword or tsurugi in a bones file (if the artifact has already been created), or by wishing for one.

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