Obelisks are tall stone structures that have always existed in the Might and Magic universe and serve the player in various ways.
In Heroes of Might and Magic, the obelisk reveals a piece of the puzzle map so the player can find the legendary Grail, or in the first two Heroes games, the ultimate artifacts.)
There is an unlimited number as to how many Obelisks may exist on a map, which can delay the discovery of the Grail significantly.
In Heroes III Obelisks can appear in many different colors. According to the map editor each color is tied to a certain terrain type (for example: green for dirt terrain, purple for snow, yellow for lava, etc). This is purely visual and each obelisk if functionally identical.