Josep Gavinho:
ooc: I don't know how to decide which square(s) you occupy or not, but the radius of your aura doesn't change when you climb on a saddle.
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See, that's the weird thing about being mounted is that it technically does.
Because i'm mounted, i treat every space my mount occupies as a space I also occupy.
https://www.d20pfsrd.com/gamem.../#TOC-Mounted-Combat
And then, because i now treat every space my mount occupies as a space of my own, my auras and "centered on you" effects have their effective area increased.
http://paizo.com/paizo/faq/v5748nruor1fm#v5748eaic9t0g
So, for determining what spaces latro occupies and how "centered on you" effects work, treat Latro as a large creature.
It's super weird, but that's how the rules go.
So when Josep casts that spell, he's within Latro's Source Severance field as well as within his AoO range.
And yes, that means if Latro was riding around on a flying whale as his mount, Latro would, for some inexplicable reason, occupy every space the flying whale occupied. I don't think we'll ever have to worry about something like that though.
change of subject, I'm guessing that concentration check was to not provoke the AoO against Latro, but Josep also needs a concentration check to even cast the spell while being grappled as well as he is still being grappled by the Wall of Bone at the start of his turn and thats a DC 27 check (10 + 12 CMD + 5)
More over, because Josep is within Latro's Source Severance field when he casts the spell, His effective caster level is reduced by either 5 or 11. Not that either of those numbers would make a difference on your Prescience roll used for concentration.
However, because i thoroughly despise Wall of Force, i'm just going to say because you forgot to roll that specific concentration check, it counts as a failed roll.
You can decide if you want to stand as a move action and Provoke and AoO from me or use the move action for something else.