Obelisks are tall stone structures that exist 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.
Anywhere from 1 to 48 obelisks can be placed on a map, and the more obelisks there are the slower the puzzle map will be revealed.
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 is functionally identical.
Heroes IV replaced the Obelisks with Oracles, that sometimes lead the way to artifacts or resources instead of the Grail. Oracles of different colors can show up in the same scenario, leading the way to different treasures. There are twelve different colors for oracles: gold, bronze, silver, white, brown, orange, yellow, blue, green, purple, red, and black.
In Heroes VI, Obelisks do not exist to reveal the puzzle map. However, an Obelisk was added in the Shades of Darkness expansion as an objective in the mission "The Pall of a Past World".