Source: Guildmaster's Guide to Ravnica

Wondrous item, rarity by keyrune (requires attunement by a member of the relevant guild)

Associated with a particular guild, a guild keyrune is a ceremonial, stylized key, about 1 foot long, made from carved stone. Not a literal key, the item is a badge of authority that gives its bearer access to privileged places in its guild's headquarters and outposts. At the DM's discretion, a character might be given a keyrune upon attaining a renown score of 25 in their guild.

When you use an action to speak the item's command word and place the keyrune on the ground in an unoccupied space within 5 feet of you, the keyrune transforms into a creature. If there isn't enough space for the creature, the keyrune doesn't transform. See the Monster Manual for the creature's stat block — the name of which is given in bold in the keyrune's description — unless you're directed to chapter 6 of this book instead.

The creature is friendly to you, your companions, and other members of your guild (unless those guild members are hostile to you). It understands your languages and obeys your spoken commands. If you issue no commands, the creature takes the Dodge action and moves to avoid danger.

The creature exists for a duration specific to each keyrune. At the end of the duration, the creature reverts to its keyrune form. It reverts early if it drops to 0 hit points or if you use an action to speak the command word again while touching it. When the creature reverts to its keyrune form, it can't transform again until 36 hours have passed.

Azorius Keyrune (Rare). This keyrune is carved from white marble and lapis lazuli to resemble a noble bird of prey. It can become a giant eagle for up to 1 hour. While the transformed eagle is within l mile of you, you can communicate with it telepathically. As an action, you can see through the eagle's eyes and hear what it hears until the start of your next turn, and you gain the benefit of its keen sight. During this time, you are deaf and blind with regard to your own senses.

Boros Keyrune (Rare). Carved from red sandstone with white granite elements to resemble a member of the Boros Legion, this keyrune can become a veteran (human) for up to 8 hours. In addition to fighting on your behalf, this veteran cheerfully offers tactical advice, which is usually sound. Anyone who talks with the transformed keyrune or examines it closely can easily recognize that it is an artificial human.

Dimir Keyrune (Very Rare). This keyrune, carved from black stone accented with steel, resembles a stylized horror. On command, it transforms into an intellect devourer that resembles the Dimir guild symbol, with six bladelike legs. The creature exists for up to 24 hours. During that time, it pursues only a single mission you give it — usually an assignment to take over someone's body, either to impersonate that person for a brief time or to extract secrets from their mind. When the mission is complete, the creature returns to you, reports its success, and reverts to its keyrune form.

Golgari Keyrune (Very Rare). Made from deep green jade with black veins, this keyrune has an insectile shape. It can transform into a giant scorpion for up to 6 hours. The scorpion has an Intelligence of 4 and can communicate with you telepathically while it is within 60 feet of you, though its messages are largely limited to describing the passage of potential prey.

Gruul Keyrune (Rare). This crude keyrune is cobbled together from bits of rubble, broken glass, bone, and animal hair. One end resembles a horned beast. On command, the keyrune transforms into a ceratok, a horned creature much like a rhinoceros (and with the same statistics). It remains in its ceratok form for 1 hour.

Izzet Keyrune (Rare). Formed of carved and polished red and blue stone, the keyrune includes bits of cable and wire. One end resembles a humanlike head, suggesting the jagged elemental form of the galvanice weird (see chapter 6 for the stat block) that it can become for a duration of 3 hours. In this form, it will serve you as a bodyguard, lift and carry things for you, act as a test subject for your experiments, or aid you in any other way that its capabilities allow.

Orzhov Keyrune (Rare). This keyrune is carved from white marble with veins of black. The end is shaped like a thrull's head, with a gold faceplate affixed. On command, the keyrune transforms into a winged thrull (see chapter 6 for the stat block) for up to 2 hours. If you don't come from an Orzhov oligarch family, it serves you grudgingly, clownishly aping your movements and mannerisms while carrying out your orders.

Rakdos Keyrune (Uncommon). This dark granite keyrune is marbled with scarlet veins and carved with the leering visage of a mischievous demon. When activated, it transforms into a cackler (see chapter 6 for the stat block) for up to 1 hour.

Selesnya Keyrune (Rare). Carved from white and green marble in the shape of a wolf's head, this keyrune transforms into a dire wolf. The wolf persists for 8 hours. Its Intelligence is 6, and it understands Elvish and Sylvan but can't speak those languages. While it is within 1 mile of you. you can communicate with each other telepathically.

Simic Keyrune (Uncommon). This keyrune is assembled from coral, mother-of pearl, and chrome and adorned with the spirals and curves characteristic of Simic ornamentation. The head resembles the shell of a sea creature. On command, the keyrune turns into a category 1 krasis (see chapter 6 for the stat block) that has the Grabber and Stabilizing Legs adaptations. The transformation lasts for up to 5 hours.