Summoning Artifact

Some Artifacts Of Doom are dangerous because they are evilly sentient and want to subvert you to their ends. Others are dangerous because they are keys to open a portal to some place of big bad nastiness, whether it be Hell or whatever place Eldritch Abominations come from, or because they have the power to summon the big bad nastiness into this world. This trope is about these kinds of artifacts.

Many Tomes Of Eldritch Lore are also artifacts of this kind, unleashing any number of nasty things upon the world when some poor Genre Blind fool makes the mistake of reading the book in question aloud. Other artifacts might require some other means of activation, especially if they're being used as the linchpin of some Big Bad's Evil Plan.

While this is primarily a Horror Tropes, examples that aren't malevolent can also be included here.

See also Artifact of Doom, Hell Gate, and Summoning Ritual. If the artifact in question is intended to summon a specific friend, as opposed to some deific supernatural entity, that's Just Whistle.

Anime and Manga

 * The Behelits from Berserk, which act as a key to the Astral Realm where the Godhand live, and are activated when their bearers cross the Despair Event Horizon. Normal Behelits are used by humans to become Apostles, while the Crimson Behelit, which appears once every 216 years, is used by its bearer to, and in both cases, a sacrifice is required.

Film

 * The Necronomicon Ex Mortis (not to be confused with the Necronomicon associated with the Cthulhu Mythos), which was accidentally used by Ash from the Evil Dead series to summon some big bad nastiness from Hell which proceeds to mess up his life in ugly fashion.
 * The apartment building in Ghostbusters, which acted as a gateway to the realm of Gozer the Gozerian.

Literature

 * The Lemarchand's Boxes from Clive Barker's horror stories, the most famous one being the Lament Configuration from the Hellraiser series, which acts as a key to the realm of the Cenobites when somebody solves it.
 * William Hope Hodgson's Carnacki, the Ghost-Finder stories. In "Gateway of the Monster," the title being (a giant strangling hand) enters our world through the Luck Ring of the Andersons - a small pentagonal ring. It was originally activated by being worn, but continued to summon the monster afterward.
 * The Shining Trapezohedron in H.P. Lovecraft's Haunter in the Dark summons the titular Haunter, an avatar of Nyarlathotep (it also gives you visions on the Outer Void and of its own history, but summoning an Eldritch Abomination is clearly its main function).
 * Robert Asprin's Thieves' World, Poul Anderson's short story "The Gate of the Flying Knives". The title gate is a large parchment 8 feet long and 4 feet wide. Through it, sikkintairs (dragon-like monsters controlled by the Ilsig gods) can be summoned to carry off or kill people.
 * Michael Moorcock's Swords Trilogy. Corum Jhaelen Irsei has a hand cut off and one eye put out. He is given two artifacts to replace his lost body parts, the Hand of Kwll and the Eye of Rhynn. With the Eye he can see into an other-dimensional cave, and with the Hand he can beckon the inhabitants of the cave to come to him and fight his enemies. Anyone killed by the summoned creatures replaces them in the cave for the next summoning.
 * Thoth-Amon's Ring of Set from the first Conan the Barbarian story, "The Phoenix on the Sword," which the sorcerer used to call down an Eldritch Abomination to destroy his tormentor and everyone with him.

Web Comics

 * The "Grimoire for Summoning the Zoologically Dubious" from Homestuck.
 * The strange program that  definitely also counts as one of these.

Anime and Manga

 * The Dragon Balls can be used to summon the Eternal Dragon, who will grant one wish.

Literature

 * Possibly derived from Lohengrin (see Theatre, below) (as C. S. Lewis was a big Wagner fan (though it may come from Astolfo's horn in Orlando Furioso -- or both), Queen Susan's horn in Prince Caspian is blown to summon aid to the army of the Old Narnians.
 * The Wheel of Time has the Horn of Valere, an artifact which summons the great Heroes of the Ages to come and fight for the one who blew the Horn.
 * But they aren't necessarily good. They're tied to fight for whoever blows the horn, not for good. So it's sort of borderline...

Live Action TV

 * The Dragon Dagger (A dagger/flute weapon) from Mighty Morphin Power Rangers/Kyouryuu Sentai Zyuranger. Playing it summons and directs Dragonzord/Dragon Caesar.
 * Power Rangers Ninja Storm has the Mammoth Zord controlled via guitar.

Tabletop Games

 * The first and subsequent editions of Dungeons and Dragons had the Horns of Valhalla, that could summon heroes from Asgard to fight for the possessor. Of course, if you didn't meet the requirements of being able to use them, the summoned berserkers would turn on you.

Theatre

 * In Richard Wagner's Lohengrin, the eponymous Swan Knight, forced to leave his wife, gives her a horn to give to her brother if she ever sees him again, promising that the horn will bring aid in danger (presumably when blown).

Video Games

 * There are three Summoning Artifacts in Tales of Monkey Island Chapter 2: The Siege of Spinner Cay, all of which can be combined into one Summoning Ball Artifact used to summon the Sea Creatures to help Guybrush find the alleged resting place of La Esponja Grande.