The Grail – what is it?

Have you ever watched the cartoon "Hedgehog in the Fog"? ;) Sorry, but the link you gave me is also unclear. Here's a quote from it: "Necropolis is a very cool Castle, and if there's a Grail there, it's a great game to play as a Necromancer." Where is the main point here – what does the Grail give in Necropolis?

Added 2 minutes later
Here, I've written down what the Grail does in different Castles:

Castle-(Colossus)-Increases the morale of all friendly heroes by 2 points
Bastion-(Spirit Guardian)-Increases the luck of all friendly heroes by 2 points
Tower-(Sky Ship)-Reveals the map of the entire world and temporarily increases the knowledge of the garrison hero by 15 during a siege
Dungeon-(Earth Guardian)-Increases the magic power of the garrison hero by 12 points during a siege
Inferno-(God of Fire)-Every week becomes a week of demons
Necropolis-(Soul Prison)-Increases the necromancy skill of all friendly necromancers by 20%
Fortress-(Monument to the Gods of War)-Increases the attack of the garrison hero by 20 points during a siege
Fortress-(Carnivorous Plant)-Increases the attack and defense of the garrison hero by 10 points during a siege
Conflux-(Rainbow)-Gives all spells to the Mage Guild of that city