Sheterra magic gate?
 

Did a search, found nothing, so sorry if this has been asked before but I couldn't find where.

There's a magic...gate thing right when you enter Sheterra. I noticed an enemy behind it. Is it a storyline thing to open it up, or did I miss something on Sheterra itself?

I'm pretty sure that this is Red Scrounger from the questline "Red Scrounger and the March of the Orcs". To get there you must have followed the complete questline and then an orc named Barlok should await you in Sheterra right where you beach your ship. He tells you something about "lights". After this you should be able to sail your ship to those lights above the water, that looks like a magic barrier or teleporter.

So check your quest log for the questline if you should look for Barlok. If not, try to follow it until this point, then come back to Sheterra and you should have the possiblity to fight Red Scrounger.

BR

J.B.

The quest starts by speaking to the orc on debir island, if you rejected to help that original orc the quest gets canceled I believe