It's really hard to work out what exactly is happening with the ability but it's definately bugged in some way, my theory is that every space it hits, it also damage units within 2 spaces as if every cannon procs fire blast. Only reason I think this though is because when I used Tremir he was sometimes able to hit units outside of the AoE and even seen the same unit take 2x/3x procs from the ability.
That's because it DOES do that? Area Effect 2, Range 5: For each friendly real Tortun (up to 5), a random space is targeted by an AE1 Cannon Ball effect. Units affected by the Cannon Ball effect take 10 Physical damage (this damage ignores DEF). So it fires, targets 1 square, but that square, and adjacent tiles (AE1) also get hit.
Oh wow, I feel dumb now. Thought it's been bugged for years. Quite suprised people don't use tortuns just for that ability now.
Now it makes sense. I was reading it like AE1 was just 1 square for some reason. Time to build tortuns
