From The Heroes of Might and Magic III wiki

Artifacts equipped in this slot usually give a bonus to Knowledge, but there are a few exceptions. The artifacts equipped in this slot are:

• Helm of the Alabaster Unicorn
• Skull Helmet
• Helm of Chaos
• Crown of the Supreme Magi
• Hellstorm Helmet
• Crown of Dragontooth
• Helm of Heavenly Enlightenment
• Thunder Helmet
• Sea Captain's Hat
• Spellbinder's Hat
• Admiral's Hat