My interpretation is slightly different -Saeviomagy said:As far as I can work out, and using my suggestions above:
A pit fiend hits you with all it's attacks, then it's tail. It then starts a grapple with a free action, makes a grapple check, wins, inflicts tail damage, then inflicts the constrict damage.
It can either
a) do iterative grapple checks at +30/+25/+20/+15, causing it's tail attack damage PLUS its constrict damage.
b) use those iterative grapple checks for other things, and cause just the constrict damage (ie - it can pin + constrict)
c) use it's full attack routine (2 wings, 2 claws, bite) minus the tail (which is grappling), with all attacks at -4, and only against the grappled subject
d) do other stuff as in my original post.
When the tail attacks hits and does damage, roll the free grapple check. If the pit fiend wins, it has established a hold, but does not constrict until the next round.
On its initiative next round, it can either
a) do iterative grapple checks at +35/+30/+25/+20. If it wins any of those checks, it does tail damage + constrict damage.
b) do iterative grapple checks at +15/+10/+5/+0. If it wins any of those checks, it does the same damage as above. It also can threaten other squares and attack other opponents with everything except its tail.
c) attack the grappled opponent with all of its natural weapons at -4 to hit. Any hits do normal damage with no constrict damage added.
d) use a spell-like ability with a successful concentration check (DC 20+level)
e) move if the grappled opponent weighs less than 25000 lbs, or fly if the opponent weighs less than 5000 lbs. Due to improved grab, no grapple check is required. The movement rate is dependent on the weight.
f) do iterative grapple checks according to option a or b above to pin. I'm not sure about applying constrict damage if the pit fiend succeeds in a pin, but as soon as it pins, it can use option a or b to do damage with the rest of the iterative grapple checks. Pinning is primarily to help others hit the grappled opponent, as he now has a 0 dex, -4 AC, and sneak attack damage is allowed. Plus the pit fiend can prevent the pinned opponent from speaking and/or use disarm attempts to grab well secured items on the opponent.
g) do iterative grapple checks according to option a or b above to use the opponent's light weapon against him. Each time it wins any of the checks, it can roll to hit at -4. If it hits, it does the damage of the light weapon plus the pit fiend's strength bonus (+13).