Recently, I have been thinking about flying creatures with the grab ability, and I have some questions.
Scenario: A flying creature with 15 feet reach flies 20 feet above the ground and attacks a medium creature without flight with a natural weapon with the grab ability, both hitting and succeeding on the grapple check.
1; The Grapple rules say that the grappled would now be placed adjacent to the grappler, which would be in the air. Is this treated the same way as succeeding on a check to maintain a grapple and moving the creature into a hazardous environment, therefore immediately giving the grappled a check to break loose?
1a; Is carrying capacity relevant here?
1b; What is treated as "adjacent"? Directly underneath the grappler, at the same heght as the grappler, or any direction, including above the grappler?
Ok, so now the grappler and the grappled are in the air.
2; Is it possible to Pin or Tie Up a grappled creature in the air?
2a; If the grappled breaks free from the grapple, would falling provoke an Attack of Opportunity, possibly allowing the grappler to grapple again with the grab ability?
2b; if the grappled creature hit the ground and fell prone, and the grappler succeeded on another grapple attempt, would the grappled creature remain prone in the air?
1. I would say yes - you can't place him over a pit (depth is not specified, so you can't place him over a 500' deep pit nor over a 5' deep pit). Likewise, it seems pretty much exactly the same thing to haul him up into the air - he's over a really wide pit.
1a. Not to the grapple, but it should be relevant to determining the encumbrance of the flyer and determining its movement rate.
1b. In two dimensions, like most fights on a battlemap, adjacent means any square that shares a border or corner with any square that you occupy. In three dimensions, this definitely applies vertically (above and below) as well as horizontally. There is some argument that you need to choose a space between the grappler and the victim, but the RAW doesn't actually say that, so ANY adjacent space is allowed, even if that makes no sense (I think most GMs apply common sense here).
2. Yes. Nothing in the grapple rules says you must be on the ground.
2a. Leaving a threatened space provokes. It doesn't matter if you choose to make the move, or you are moved against your will, such as on a galloping horse, swept down a river, or falling. Note that some special abilities say that if you use them to force an opponent to move then his movement does not provoke - those are special cases but for everything else, the rule that leaving a threatened space provokes is the default.
2b. I don't think prone applies in the air: "Prone: The character is lying on the ground.". Most of the problem with being prone isn't that your body is horizontal instead of vertical, it's that the ground impedes what you want to do. In the air, the ground isn't much of an issue, except for that sickening crunch at the bottom of a fall...
1: No. you did not maintain a grapple and use a move action. Now, round 2, after you grabbed him, if you are still up in the air, yes, it would be hazardous and he would get the free escape check with a +4 bonus.
On successful grab he would be moved up to a square adjacent to you.
1b: I'd say nearest square that is adjacent. In this case a square below the flying grappler.
2a) Falling is not a move action, so no it would not. Relevant rules here:
"Provoking an Attack of Opportunity: Two kinds of actions can provoke attacks of opportunity: moving out of a threatened square and performing certain actions within a threatened square."
Unless an ability specifically forces you to provoke (such as being tripped by someone with the greater trip feat) you cannot provoke without taking an action. You did not take a move action to start falling, and therefore you do not provoke.
2b) This is the murkiest of what you've asked. I agree that prone requires a surface to be prone upon. I don't think you can be prone while flying. (Evidenced by creatures with a fly speed being immune to trip.)
To expand on part 2a:
Let us say I am standing 5 ft away from a 10 ft tall building with a reach weapon. Any one standing at the edge of the building is currently out of my reach (using basic math he is roughly 12 ft away). If that same character would drop to the square next to me he would pass through the square I threaten vertically. Would I then get an attack of opportunity?