Magog
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why other nerf options don't solve the problem – in my opinion, this is what Andrail was asking for ;)
Added 1 minute ago
Regarding invisibility: I think it should be made infinite (let's say 2-3 turns, then the next use), because it only grants immunity from direct attacks, nothing more. I don't know why it's so highly regarded, but the Crystal of Secrecy works very well against invisibility, and area-of-effect spells deal damage to invisible units.
You forget that no one is going to take them to the endgame. But they greatly boost the Warlock in early-game skirmishes with minimal losses.