Source: Guildmaster's Guide to Ravnica

Wondrous item, uncommon (requires attunement by a sorcerer, warlock, or wizard)

Innovation is a dangerous pursuit, at least the way the mages of the Izzet League engage in it. As protection against the risk of an experiment going awry, they have developed a device to help channel and control their magic. This apparatus is a collection of leather straps, flexible tubing, glass cylinders, and plates, bracers, and fittings made from a magic-infused metal alloy called mizzium, all assembled into a harness. The item weighs 8 pounds.

While you are wearing the mizzium apparatus, you can use it as an arcane focus. In addition, you can attempt to cast a spell that you do not know or have prepared. The spell you choose must be on your class's spell list and of a level for which you have a spell slot, and you must provide the spell's components.

You expend a spell slot to cast the spell as normal, but before resolving it you must make an Intelligence (Arcana) check. The DC is 10 + twice the level of the spell slot you expend to cast the spell.

On a successful check, you cast the spell as normal, using your spell save DC and spellcasting ability modifier. On a failed check, you cast a different spell from the one you intended. Randomly determine the spell you cast by rolling on the table for the level of the spell slot you expended. If the slot is 6th level or higher, roll on the table for 5th-level spells.

If you try to cast a cantrip you don't know, the DC for the Intelligence (Arcana) check is 10, and on a failed check, there is no effect.

1st-Level Spells
d6 Spell
1 Burning Hands
2 Chaos Bolt
3 Color Spray
4 Faerie Fire
5 Fog Cloud
6 Thunderwave
2nd-Level Spells
d6 Spell
1 Blur
2 Gust of Wind
3 Heat Metal
4 Melf's Acid Arrow
5 Scorching Ray
6 Shatter
3rd-Level Spells
d6 Spell
1 Fear
2 Feign Death
3 Fireball
4 Gaseous Form
5 Sleet Storm
6 Stinking Cloud
4th-Level Spells
d4 Spell
1 Confusion
2 Conjure Minor Elementals
3 Evard's Black Tentacles
4 Ice Storm
5th-Level Spells
d4 Spell
1 Animate Objects
2 Cloudkill
3 Cone of Cold
4 Flame Strike