Loules
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Letos is great on large maps, and Synitar with the Warlord skill is very convenient on small maps. On small maps, you might not even find the spells of the Necropolis faction. Erinn is a weak hero; Iranna is better.
Well, I don't really know, but if we're talking about a hero in the late game, it's Erinn (or Irbeth).
The ability of Synitar is usually not helpful in a prolonged game (fortunately, the Necropolis troops are fragile and unlikely to survive long enough for us, with the artifacts we have, to run out of mana).
But even if mana is needed, Irbeth is more advantageous, because by spending a day, she allows you to gain about a hundred mana above the maximum (which will allow you to at least put up some resistance against demons that drain mana).
In my opinion, the most interesting heroes in the late game are Letos and Erinn. However, Letos will be limited by the damage of the plague (which in the late game will be a couple of hundred - not bad, but not strong enough to be considered overpowered), while Erinn's damage is not limited. Plus, he combines well with a build focused on a mage-chaos hero, as it allows him to deal additional damage without requiring complex conditions for execution (several melee units, and even in the right positions), as with Sorgan.
In my opinion, if there are no plans for serious mana problems due to some features of the enemy, Erinn is potentially the strongest in terms of chaos magic in the League.