From The Heroes of Might and Magic III wiki

This is a combination artifact.

Combination artifacts were introduced in the expansion Shadow of Death,
and are only available when it is installed.

In addition to the other bonuses the Ring of the Magi will add 50 rounds to spell duration (56 rounds in total, in addition to initial duration).

Requires the following artifacts to create:

• Collar of Conjuring
• Ring of Conjuring
• Cape of Conjuring

You cannot purchase combination artifacts in Artifact Merchants. Cost is derived from sell price.

Basic Cost: 15000 gold