From The Heroes of Might and Magic III wiki

Grail building for the Conflux.

To build this structure, find and bring the Grail artifact to a Conflux 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 Aurora Borealis in particular also:

• Fills the town's Mage Guild (no matter the level) with all unbanned spells. (Excluding Titan's Lightning Bolt)

[edit] Comments

The best Grail building, all heroes that enter the town get all spells there is in the game. However, in games with lots of towns all the important spells may be available even without Aurora Borealis.