Mapping and the Atlas
Although the main storyline ends at Act 10, hundreds (if not thousands!) of hours of fun still lay ahead, by way of a special kind of item called maps.
Maps are items that open up brand new areas through the Map Device, located in the Epilogue area. High level monsters in the last few Acts can drop the earliest maps, but higher tier maps can only be found within other maps. Like other items, maps can be normal, magic, rare or unique, and have a wide pool of mods that affect both the difficulty and reward of the areas they open. Carefully crafting your maps for challenges that suit your build is an important part of Path of Exile's map system. It's always worth checking the mods on a map before you try to complete it. Some of them can be very punishing!
Using the Atlas
You can view which maps you have completed in your Atlas (default hotkey G). You can also further influence the properties of each map by applying a sextant to the Atlas icon. Sextants last for a limited number of maps and affect every map within its radius. While in a map, enemies can drop any maps of a lower tier that you have previously completed, as well as maps of a higher tier that the map you are in is connected to. Map bosses have an especially high chance to drop a map. Maps also have a bonus objective; usually to complete the map as a specific rarity. Completing this bonus objective increases the chances of finding higher tier maps. The higher the map tier, however, the harder it is to obtain even higher tier maps.
Later, you'll find Shaper's Orbs which let you upgrade the tier of a map, increasing the difficulty and levels of the monsters within. Most players aim to work their way towards the centre of the Atlas in the hopes of challenging The Shaper, a very difficult but rewarding boss.
Shaper and Elder
You may soon find strange patterns on your Atlas, such as swirling tentacles or glittering stars. These are indicators of whose influence those maps are under. Maps concealed by tentacles are under the influence of an entity called The Elder, while maps masked by stars are under the influence of The Shaper. Each of these entities augment the maps they are influencing, either by adding extra monsters or making the boss fights more difficult. Completing a map that is influenced by one of these entities will remove the influence and may cause the other entity to influence that map instead.
Completing other maps may also grow an entity's influence, though since the Shaper and Elder are at war with each other, you may not always get the outcome you want. If you want to grow The Elder's influence, your best bet may be to target Shaper-influenced areas or areas with no influence at all. As The Elder's influence grows, you may find that several maps become inhabited by The Elder's guardians. Completing these maps involves tough boss encounters, but if you manage to do it, they may lead to The Elder itself. Monsters in Shaper- or Elder-influenced maps can drop special versions of items that are also Elder- or Shaper-influenced. These items can roll special (and often very powerful) mods that are not ordinarily available.