Shifter (scorpion aspect) poison

[Sintog]

When do you get access to the poison? I could see at level 4 as it lists sting. But then later at level 15 it calls out using the poison specifically for AoO, which suggests you don't get the poison until lvl 15 and when you do get it, it only works in specific circumstances.

[Lelomenia]

15th.

[blahpers]

You get the 1d6 sting at 4th level, but you don't get the poison until 15th, and then only under the listed circumstances.