With the 'orc mage', you are referring to Dergiab the ogre in the orge caves along the north road, correct?
In the cave area directly around his position, there are four doors. Two metal ones connecting the place where you fight him to the rest of the corridors in the area, and two wooden ones built into the little homes. Now, the metal ones have hardness 10 and the wooden ones hardness 5. "door: resists damage (5)" would suggest you're trying to beat down the wooden ones.
However, these cannot be destroyed in any way, and opening them is impossible without a key, which does not exist as no key has been assigned to the lock. Basically this means the door can't be opened. Which is as it should be, as no area has been created for the door to lead to in any case.
With the door being indestructable and unopenable, your mage shouldn't have been able to pass through in the first place. Also, he shouldn't have had any reason to do so in the first place, as his orders are simply to attack on sight. Which means there's a bug at work here.
Now, you can't open the door, and you can't kill Dergiab. This means you can't complete the quest.
You could possibly use the "`" key (top left on most keyboards) with a command to spawn an item in your own inventory, but I've not yet had the need to use it and consequently haven't looked up what command that might be. I also am not sure if it is at all possible to obtain the item that way.
Alternatively, you could save the game and start a multiplayer session, with you playing the DM. Jump to the proper area and create the item on the ground. Save, and load that save in singleplayer mode. This, too, I haven't required often enough for me to know the details of.
In any case, the name of the item you want to get is "Dergiab's Head". It has tag "M2Q5DERGIABHEAD" and blueprint "m2q5dergiabhead". Experiment a bit, or do a search for the details of importing items to single player games. Or, wait around for someone else on IW to provide better details
