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 regular bonuses the Titan's Thunder gives the hero the Titan's Lightning Bolt that causes 600 points of damage and does not cost any spell points. If the hero does not have a spell book, then one will be permanently added.

Requires the following artifacts to create:

• Thunder Helmet
• Titan's Gladius
• Titan's Cuirass
• Sentinel's Shield

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

Basic Cost: 200000 gold