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I just can't believe it! Who do they want to replace?! The Minotaurs and Hydras! These are some of the coolest veteran units in the game! If you think about it, the Dark Elves didn't really add a new Castle; they just added three units – lizards, assassins, and the Shadow Lord. The rest – Furies (harpies with their wings cut off but with added muscle), Minotaurs, Hydras, and, of course, Black Dragons – all of this is a legacy of the Warlock/Overlord. (The Hydras were temporarily moved to the Swamp in Heroes 3, though.) The Castle, by the way, remains a cave.
Now, let's talk about their practical combat application:
1) The Hydra is excellent at absorbing damage and has regeneration. If the Dark Elf has high Strength and Chaos magic (as befits a Dark Elf hero), you can use Hydras in moderately difficult battles against the computer (like taking lightly guarded Castles and fighting particularly strong neutrals). You can simply line up Hydras and blast everyone with magic. Against a human player, if they don't have the Amulet of War (which is easy to check) and you have enough money, you can hunt down the main hero – hit them with powerful spells and then retreat, buying back your Hydras.
In tough battles (against the enemy's main army), Hydras are good for protecting ranged units.
2) The debate about replacing the Minotaurs is just a bad joke! They are an indispensable unit in tough battles, capable of causing so much trouble on their own that the enemy will cry. And paired with the lizards, they simply work wonders on the battlefield. Plus, they make excellent phantoms.
So, NO ONE in the Dark Elf army needs to be replaced; they are already strong (there is a weak point, though – Demons with their Imps/Devils). And those who say "Dark Elves are the weakest" are the ones who haven't faced a serious Dark Elf army!