[MENTION=913]Schmoe[/MENTION], your post indicates you are moving. Is this correct? Are you positioning yourself between the Goblin and the southwest tunnel? I ask because you would be exposing yourself to an Attack of Opportunity by doing so. So before I adjudicate, I have to make sure you are aware of this and that this is what you want to do. Of course, you don't have to move; I just need clarification based on what you have in your IC post.
In a fight, everyone is constantly watching for enemies to drop their guard. You can rarely move heedlessly past your foes without putting yourself in danger; doing so provokes an opportunity attack.
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack interrupts the provoking creature's movement, occurring right before the creature leaves your reach.
You can avoid provoking an opportunity attack by taking the Disengage action. You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an enemy.
Most creatures have a 5-foot reach and can thus attack targets within 5 feet of them when making a melee attack. Certain creatures (typically those larger than Medium) have melee attacks with a greater reach than 5 feet, as noted in their descriptions.
As you wouldn't be moving OUT of the Goblin's reach, the AoO shouldn't apply, right?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.