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What does the special building for the Dark Elves give? The one that looks like a huge crystal?

This unique structure of the Dungeon faction is called the Elemental Altar (or its improved version, the Primal Elemental Altar). It randomly displays the classic elements that make up a creature. Creatures with opposing elements (fire-water, air-earth) in the Dungeon army deal additional damage to their opponents. Upgrading the Altar increases the damage dealt in this way by 10% for each such structure, and this advantage becomes particularly noticeable when building 3 or more improved Altars. If a Dungeon hero gains the ultimate ability Elemental Fury, it will further increase the damage from elemental chains by 100%.