Occasionally in combat, certain tiles will show an exclamation mark icon on them (as shown below). What do these icons mean, and what causes them to appear?
These are tiles where explosions/area-of-affect attacks are going to take place during the next attack phase.
Examples of explosions/aoe being caused: The pixie explosion attack, mortar attacks (including rainbow horse), the mortar gun used during Helmitor battle, and the mushroom (fart attack thing).
Basically any attack where it will affect multiple tiles surrounding it and puts your own characters at risk of damage. The ! warning shows up any time that an area of affect attack will be taking place. These can damage any character, friend or foe which is why they signal it being dangerous for being in the tile.
In your screenshot, basically what it's saying is the mushroom is moving to that tile, and will damage people in the surrounding tiles.
Source: Playing the game a whole lot.
1I'm not sure that your explosive theory is 100% right. Watching this video, you can see the tiles are marked with exclamation points, but I don't see any kind of explosion happening. However, the rest of your answer may be valid for why they show up.– Timmy Jim ♦Mar 1, 2017 at 22:16
1If not an explosion, it in general represents area of affect attacks. In that scenario, the wrath can misfire and while he will aim for those two enemies in his range, he can potentially hit allies who step into that range. Mar 1, 2017 at 22:18
1AoE that's more like it.– Timmy Jim ♦Mar 1, 2017 at 22:18
It's most commonly explosions such as pixie or mortar attacks but yeah AoE in a general sense. Mar 1, 2017 at 22:18
I found a Steam Forum that states that these are tiles you want to avoid. The help menu in the game describes these as "Danger".
The exclamation mark is 'look out' or 'avoid'.
A screenshot from that forum is below. "Danger" is the second one down on the second column:
1An upvote to you for finding what the icon means directly for the game, but I'm hoping to find what causes the icon to appear before accepting an answer. Mar 1, 2017 at 21:56
I can write up the reason why it shows in a minute :) Mar 1, 2017 at 22:02