Source: Guildmaster's Guide to Ravnica

Ring, uncommon (requires attunement)

This ring, adorned with the symbol of a guild, allows you to cast one spell closely associated with that guild, as shown in the Guild Signet Spells table. A guild signet is sometimes awarded to a guild member whose renown score in that guild is 5 or higher, as a reward for performing special services for the guild. Aside from its magical properties, the ring is also an indicator of the guild's recognition and favor.

A signet has 3 charges, and it regains 1d3 expended charges daily at dawn. While wearing it, you can expend 1 charge to cast the associated spell (save DC 13).

Guild Signet Spells
Guild Associated Spell
Azorious Ensnaring Strike
Boros Heroism
Dimir Disguise Self
Golgari Entangle
Gruul Compelled Duel
Izzet Chaos Bolt
Orzhov Command
Rakdos Hellish Rebuke
Selesnya Charm Person
Simic Expeditious Retreat