The Grail in Olden Era retains its role as a powerful reward for exploration, but the system has been changed so that the Grail Sanctuary appears more frequently in ordinary matches and influences the game rather than remaining a rare late-game goal. Source: Devlog #12.
How to obtain the Grail
To obtain the Grail, a hero must enter a special battle inside a Mirage object. In the Mirage, they fight against a copy of their own army, and the army inside is always stronger. After victory, the Grail is added to the player's pool of available resources.
Why the battle is difficult
The hero's copy in the Mirage has a much stronger army, significantly increased primary stats, and can use all spells learned by the hero. The developers have also added mechanics against exploits such as "1 troglodyte + magic arrow". Since the army composition changes from game to game, encountering a Mirage should feel different every time.
Building the Grail Sanctuary
After obtaining the Grail, the player can go through their towns and choose where to build a Grail Sanctuary. Unlike Heroes III and IV, the Grail Sanctuary is in the building list from day one, and its bonuses can be read immediately. Construction in the chosen town consumes the Grail from the resource pool.
Strategic Significance
The bonuses depend on the faction of the town where the sanctuary is built. If an opponent captures a town with a Grail Sanctuary, they receive the sanctuary's bonuses as long as they control that town. Therefore, after obtaining the Grail, it is important not only to choose the best bonus but also to protect the chosen town.