Wyverns and thunder fire cannons are super common in my group so I've never really seen him shine. I've seen him summon a bloodthirster, but the bloodthirster never got into combat so it was pretty unimpressive.
Summoning more heralds isn't all that useful, so you really want incursion or possession. That's a one in three chance of getting something good. So in a 6 turn game it happens twice, but if it happens later in the game you may never end up getting your new units to do anything.
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