The monolith is an adventure map structure appearing in every Heroes of Might and Magic game except Heroes I. Going into a monolith will take the hero to another monolith. There are two kinds of monoliths: one-way and two-way.
In Heroes II, the structure is called the stone liths, and only the two-way version exists. In Heroes IV, it's called the portal, and in Heroes VI, it's called the spirit gate. It is called the monolith in both Heroes III and Heroes V.
Heroes can enter through any two-way monolith and exit through any other with the same color. In most games, if there is more than two two-way monoliths of the same color on the map, they will exit a random one - it can be any monolith of that color, except the one they entered. In Heroes IV, the hero is allowed to choose which portal to exit.
In Heroes III two-way monoliths appear as stone doorways with stone wings on top of them. They always have some form of magical visual effect in the middle of the doorway. The Armageddon's Blade expansion added some new models for two-way monoliths.
In Heroes IV two-way monoliths look like stone humanoid heads with horns and glowing eyes. The mouth part acts as the portal and has a glowing magical visual effect in it.
Two different structures make up the one-way monolith: the entrance and the exit. Entering a one-way entrance will take the hero to an exit matching the entrance's color. Like the two-way monolith, when there is more than one exit, the hero will be taken to a random one, except in Heroes IV, where they are allowed to choose.
In Heroes III one-way monolith entrances appear as stone doorways without the stone wings. They always have some form of magical visual effect in the middle of the doorway. One-way exits look similar to the entrances but they always lack the visual effects of the entrances. The Armageddon's Blade expansion added some new models for one-way monoliths.
In Heroes IV one-way monoliths look like some sort of dragon statues. The monolith entrance always has a glowing magical visual effect in the middle of the doorway. The monolith exit looks identical to the entrance but it lacks the glowing visual effect.