I'm playing a wizard that uses conjuration for a lot of his offensive magic. I was looking at my spell list and was wondering what happens to a creature who fails a save against Aqueous Orb (and is thus stuck inside it), and then I move that Orb into the area of Black Tentacles. Can I move the orb out of the tentacles? Does the creature stay with the tentacles or the orb? Can I repeatedly use my Orb to drop creatures in the Black Tentacles?
An aqueous orb can move into black tentacles fine, as it can move over obstacles less than 10' high. A creature caught in the orb would also be subject to the tentacles (or any other area effect they move into).
You can't choose to expel someone from the orb, but I would tend to rule that if both the orb and tentacles both grab someone the tentacles would win, so sure, given enough failed saves you could use the orb to dump multiple enemies into the tentacles. It's a judgement call; it'd be best if both spells used the same CMB mechanic because then it'd be trivial to have them roll against each other to see who wins the tug of war with the victim, but since it's CMB vs Reflex save I'd tend to say the higher level spell wins out.
They gain cover against attacks from outside the aqueous orb but are considered entangled by its churning currents, takes 2d6 points of nonlethal damage at the beginning of their turn each round they remain trapped. Creatures within the orb may attempt a new Reflex save each round to escape into a random square adjacent to the aqueous orb. The orb may hold one Large creature, 4 Medium, or 16 Small or smaller creatures within it.
As aqueous orb states here target will gain cover which is a +4 to ac. And entanglement is a -4 to dex. Also from this section:
When determining the tentacles' CMB, the tentacles use your caster level as their base attack bonus and receive a +4 bonus due to their Strength and a +1 size bonus. Roll only once for the entire spell effect each round and apply the result to all creatures in the area of effect.
We can conclude that the tentacles are of large creature type (+1 to str and +4 to cmb). Therefor the caster will have to choose which creature will stay submerged if the aquous orb rolls over the tentacle to even try and attack the creature already with in. In turn will kinda make an attack useless as it will lose its entanglement status (if failed reflex save) switch to your tentacle and still have its normal ac. However a ranged attack maybe the better bet if it is trapped within as entanglement status will continue.
With that said if a house rule is to allow two spells cast by caster to combine, if both spells are in active play; (previously cast on different rounds due to length of time or round). Then sure your tentacle should be able to reach in and attack. However to balance the creature should be removed adjacent to aqueous orb and entangled by tentacle. (Or if you will the black tentacle reaches within and takes target out). Once again this would have to be gm discretion.
Every creature within the area of the spell is the target of a combat maneuver check made to grapple each round at the beginning of your turn, including the round that black tentacles is cast. It is not a reach attack.
However i did say as a house rule. If you can combine two active spells and have the aquaous orb nearby one tentacle it could reach in and grapple target. However it would remove the target from the orb to balance the mechanic of a -4 dex and another -4 to dex plus the -2 to attack rolls from both conditions does not seem fair.