From The Heroes of Might and Magic III wiki

Grail building for the Stronghold.

To build this structure, find and bring the Grail artifact to a Stronghold town, and click on the Village/Town/City Hall or Capitol, and you will be asked if this is where you want the permanent home of the Grail to be.

The Grail grants the following to all town types:

• 5000 extra gold per day
• 50% additional creature growth

The Warlords' Monument in particular also:

• Increases the Attack of a garrison hero by 20 when defending against a siege.