Yeah, that's why I was hesitant to put it in there, but I thought that in a 2-player game the heroes would be revealing 3/4 tiles per round while the Troll only will take 1, and I wanted to give him something that will make you flee from him so he can use his ability, but yeah it could be scratched that completely.
The problem that I have with that monster effect is that it doesn't stop the players for playing bad but totally punish them after the bad play was done. It kinda reminds me of 'Uber' when you stray too far from other heroes. In this case if a player go too long on straight line and the monster pop up from another hero, the players can conspire to quickly deplete all the tiles to let the single hero surrounded with only void. You may turn back if they don't kill the Troll after the Boss has pooped out, but it wouldn't be funny to that to happen.
You say it's strong, but do you think his stats are powercreeped? I just look at the other troll and make it almost the same, giving it less evasion and more defense for the reasons stated.
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