I'll be controlling a character with a breath weapon (5-foot cone) and I don't know how that works.
Is it a single 5' square? Is it three squares?
If the creature has a reach of 0 ft. does the square it's in get hit by the breath weapon? If so, can it choose not to?
What is giving you a 5' cone?
And searching around (I searched, honest) it looks like Undine can get one too. (ooze breath, yuck)
One square makes sense, but the magic chapter talks about picking a corner and counting squares, so I wanted to make sure it wasn't a semi-circle.
Any thoughts on reach? I guess if I pick a corner, it can't work on the square I'm in, so I can't spray someone who is sharing my square.
"Don't be afraid, be horribly sick instead!"
Reach has absolutely nothing to do with area effects. It only matters when attacking with weapons (manufactured or natural). Breath weapons (and spells) don't care about reach.