Path of Exile’s end-game areas are items called Maps. They grant limited access to a new world area that can contain great challenges and greater rewards. Like other items, Maps can have random mods that affect their difficulty.
In 0.9.11 there is no Maelstrom of Chaos. When you complete the (temporary) end of the Beta by finishing Act Two of Merciless difficulty, you will instead be given a waypoint to the Eternal Laboratory. This is an area that you will eventually find in Act Three, but designers are granting access to a temporary equivalent earlier so that Maps can be tested.
In the Eternal Laboratory is a device that allows you to use a Map. The Map is consumed and six portals are created that can be used to travel to the area it represents. The portals close as they are used by players. A party of six players can only enter the Map once and can only leave with six inventories full of items. A smaller group of players will have some portals spare to carry additional items out of the area or re-enter if they die. Players may choose to play alone if they want to use all six portals to complete the area.
When players enter their Map, they appear near a portal at the beginning. If they use a Portal scroll while in the area, it moves the starting portal to where they are standing. This means that when they re-enter the Map via the Eternal Laboratory (where they consume one of the available portals), they will be at the new portal location, as opposed to the initial portal location. This is convenient for bringing friends in to help with the challenges of the area.
In addition to chests and packs of random monsters, each Map is inhabited by a Unique boss monster. These monsters are very difficult and drop an increased quantity of items. Each Map base type has a consistent boss monster, so players are able to plan their build around the type of Map they’re running. For example, if the Overgrown Ruin Maps have a boss that uses Lightning Strike, then players know they need to bring a lot of lightning resistance to defeat it. Players can share information about what Unique boss they find in each Map level in the same way that they assembled a list of vendor recipes and Unique items.