This arena lies within the Highlands of Outworld. The Portal remains still, with the exception of lightning striking in the center. This gateway is very unstable and unpredictable. Shadow Priests stand guard as watchers. Where the portal actually leads to still remains a mystery. In Mortal Kombat: Shaolin Monks, the Portal is the basis of several areas in Outworld (essentially its hub). Each of these areas are generally guarded by two Shadow Priests that must be defeated before the bridge to the portal will appear.
Mortal Kombat II
In the Sega Genesis version of Mortal Kombat II, there was a blue palette-swapped version of the Portal that was used to fight the secret characters Jade, Smoke and Noob Saibot instead of fighting in Goro's Lair.
The MKII version of the Portal also reappeared in Mortal Kombat Trilogy with an added pagoda-style temple on each end of the arena. The towers from the Hidden Portal can be seen on one side of the arena (now filled with characters from MK1 to MK3 on separate towers if observed closely), with the Portal arena on the other.
Mortal Kombat: Deception
In Mortal Kombat: Deception, the Portal appears as a combination of its II and 3 incarnations. One side displays the MK3 version (complete with filled ladder spaces using portraits from MK1 to MK4) and the other shows its MKII incarnation. Unlike MKII, five Shadow Priests will watch battles between players there. Chose Your Destiny towers can also be seen. This version is very similar to the Hidden Portal.
Mortal Kombat: Shaolin Monks
The Portal is the third stage of the game Mortal Kombat: Shaolin Monks and is accessed after having removed the Tarkata hordes from the Wu Shi Academy at this stage players can access a number of portals and are sent to certain points in the game where they must stop the warriors of Shang Tsung. The main enemies of this stage are the already-known Shadow priests, who use their dark magic to try to put an end to the players as well as attempting to stop Liu Kang and Kung Lao's access to any portal. At this stage of the game you can access other areas found in the same way. To get there you have to be very fast as far as combat is concerned since Shadow Priests will have the help of other killers like the Tarkata or other enemies such as the Demon Captain and other guards of Shao Kahn.