From The Heroes of Might and Magic III wiki

Grail building for the Inferno.

To build this structure, find and bring the Grail artifact to a Inferno 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 Deity of Fire in particular also:

• Causes every week after to be the week of the Imp, with +15 growth to Imps and Familiars.

[edit] Comments

When the "Month of the Imp" that can be generated by this structure occurs, stacks of Imp will be spawned on the map.

The Grail building is usually built long after you have high level troops are in the game. You would need massive numbers of joiners to make the Imps worthwhile. A Necropolis player benefits as well.