It’s another way of doing sky boxes. Most games bake their sky box down to a cubemap and just use that straight.
A portalsky is done as a whole other unreachable area on the map, done in a much smaller scale, that the regular camera is moved to first before drawing the map proper. The downside is that you are drawing more geometry, but the upside is that the background details look like an actual part of the map.
Here’s an example from NeoTokyo, you can see the unfinished building and the radio tower from multiple angles in the map, but if you go outside the map proper, they both disappear. Instead you can see them in this other miniature map off to the side (outlined in orange)
Another instance of demonic forces being combined with UAC technology were what the research team called Revenants. At an average height of 10-11 feet, Revenants were found to be a race of giant men, whose animated corpses were responsible for a lot of construction work in Hell itself, their size and strength exploited for slave-labor. A lot of this involved heavy lifting of raw materials, resulting in their arms being outstretched. Aside from this, their muscle and skin tissue (and even the presence of internal organs) seem to be slowly, but constantly regenerating, as if Hell kept them in a state of temporary decomposition. Once through the Hell-Gate, a shroud of metal enveloped much of their shoulders and torsos, mounted with miniature rocket or fragmentation grenade launchers, some with homing capabilities. Though this high amount of firepower alone makes Revenants a force to be reckoned with, their long arms and sturdy hands have turned them into excellent fist-fighters, just as devastating in close-combat.
Note 008-01: Though many human lives have been lost to the Revenants, they have been spotted attacking their own allies, perhaps in defiant retaliation to their masters.