From The Heroes of Might and Magic III wiki

Grail building for the Tower.

To build this structure, find and bring the Grail artifact to a Tower 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 Skyship in particular also:

• Reveals the entire map.
• Increases the knowledge of any defending hero by 15 when defending against a siege.

[edit] Comments

Unlike the Lookout Tower, the Skyship can't be used to disperse the darkness generated by a Cover of Darkness.