[OoC: No it's not a surprise round. The Kobolds were alerted to you. But they are considered flatfooted until their turn. But your AoO would happen on their turn so he will have his full armor class. Sorry.]
Mardak moves to put himself in the best position [AA7*] in the middle of the tunnel. [Placing himself on line 6 would allow the Kobold to do a withdraw action and run down to Line 8 and then right past him.] The kobold knows he can't run by him without the chance of getting hit so he thrusts his spear at Mardak first in the hopes of knocking out or killing him but missed. [Rolled an 11 on my phone but then lost the post (again) when trying to use Tapatalk App.
] The Kobold then has no choice but to make a run for it so he charges past Mardak. The Hexblade swings for the critter but the little guy dodges his blade and runs into the cave.
21 Jonath [C8]
18 Tomeal [D8]
14 Mardak [Line 7 behind Kobold at A6. I don't have a letter there yet so Im calling it AA7]*
11 Nacesh [D6]
11 Kobold [B8] [Unconscious at -1]
11 Kobold [Ran 30' so he is 15' past the map.]
09 Betimi
08 Selina
[OoC: It is now Betimi and Selina's turns. Then Jonath, Tomeal, Mardak and Nacesh's turns]
[FYI: Kobolds have darkvision so it's dark in the cave. So unless someone produces some light or has a light spell (*cough* Selina), only the people with Darkvision, who I think is only Jonath and Betimi, can see. The people with Darkvison see that the Kobold ran over a small bridge and is just outside [5'] of some kind of Portcullis. You can surmise that he will likely make his 5' step and then close the Portcullis, locking you all outside. ]
Mardak moves to put himself in the best position [AA7*] in the middle of the tunnel. [Placing himself on line 6 would allow the Kobold to do a withdraw action and run down to Line 8 and then right past him.] The kobold knows he can't run by him without the chance of getting hit so he thrusts his spear at Mardak first in the hopes of knocking out or killing him but missed. [Rolled an 11 on my phone but then lost the post (again) when trying to use Tapatalk App.

21 Jonath [C8]
18 Tomeal [D8]
14 Mardak [Line 7 behind Kobold at A6. I don't have a letter there yet so Im calling it AA7]*
11 Nacesh [D6]
11 Kobold [B8] [Unconscious at -1]
11 Kobold [Ran 30' so he is 15' past the map.]
09 Betimi
08 Selina
[OoC: It is now Betimi and Selina's turns. Then Jonath, Tomeal, Mardak and Nacesh's turns]
[FYI: Kobolds have darkvision so it's dark in the cave. So unless someone produces some light or has a light spell (*cough* Selina), only the people with Darkvision, who I think is only Jonath and Betimi, can see. The people with Darkvison see that the Kobold ran over a small bridge and is just outside [5'] of some kind of Portcullis. You can surmise that he will likely make his 5' step and then close the Portcullis, locking you all outside. ]