Considering helping out on the wiki to keep it updated - in particular the changed unique items need to be updated to reflect 2.6.0 changes.
Mods and base items have been updated to 2.6 values - skill progressions on some skill gem pages may need updating or some bugs need to be fixed.
Atlas of Worlds
The Atlas of Worlds replaced the existing end-game map system as part of the Atlas of Worlds expansion, released on September 2, 2016. The Atlas is a series of linked maps that players can progress through after completing a quest with Zana, Master Cartographer.
In addition, the Atlas of Worlds also has specific "thematic" zones which enable specific base item type drops.
All maps, both base maps and unique maps are located on the Atlas of Worlds. Map drops are no longer completely random like before; instead a specific tier is chosen first and the map type is chosen from pool of maps completed for that tier or connected to the map the player is currently playing. 
Each completed map base type also gives a 1% chance to upgrade the tier of a map, with values over 100% giving a chance to increase the tier a second time. The maximum map tier however can not bypass drop restrictions, even if it would have been upgraded.
For example, a rare monster in a tier 6 map with 125% completion bonus:
- a rare monster can drop +1 maps, as such it can drop tier 1 to 7 maps
- it will only drop maps that are adjacent or already unlocked on the atlas
- a tier 5 drop would be upgraded to tier 6, with a 25% chance to be upgraded to tier 7
- a tier 6 drop would be upgraded to tier 7, but can't be upgraded any further
- a tier 7 is already the highest possible drop, and can't be upgraded any further
Non-unique pre-Atlas maps from prior to Version 2.5.0 do not count towards Atlas completion, even for maps where the map type and tier are identical to the current version. Older versions of unique maps will count towards Atlas completion though, including the bonus objective, but the Shaper's Orb does not drop in old versions (affecting Oba's Cursed TroveOba's Cursed Trove
Torture Chamber MapMap Level: 78
Map Tier: 11
Item Quantity: +40%Area has patches of desecrated ground
20 patches with Ground Effect per 100 tiles (Hidden)
Ground Effect has a radius of 10 (Hidden)
(40-50)% more Monster Life
(30-40)% increased Monster Damage
Area becomes fatal after some time"Here the soil is more deadly than any swing of the axe."
- Oba of the Karui, Conqueror of Corruption.Travel to this Map by using it in the Eternal Laboratory or a personal Map Device. Maps can only be used once., Poorjoy's AsylumPoorjoy's Asylum
Temple MapMap Level: 76
Map Tier: 9
Item Quantity: +100%
Item Rarity: +250%(140-160)% more Monster Life
(140-160)% increased Monster Damage
25% increased Monster Movement Speed
25% increased Monster Attack Speed
25% increased Monster Cast Speed
Area is a large Maze
150% increased Experience gain
Unique Boss drops (10-15) additional Rare (Life Flasks-Belts)Laughs of all the twisted
Echo through these halls of gold.
In this cold forgotten void
They wander, forever uncontrolled.Travel to this Map by using it in the Eternal Laboratory or a personal Map Device. Maps can only be used once., Death and TaxesDeath and Taxes
Necropolis MapMap Level: 79
Map Tier: 12
Item Quantity: +150%Unique Boss deals (50-60)% increased Damage
Unique Boss has (25-30)% increased Attack and Cast Speed
Unique Boss gives 10000% increased ExperienceSo the raiders danced,
leading their foe ever west,
towards certain death.Travel to this Map by using it in the Eternal Laboratory or a personal Map Device. Maps can only be used once. and The Vinktar SquareThe Vinktar Square
Courtyard MapMap Level: 77
Map Tier: 10
Item Quantity: (+100% to +150%)
Item Rarity: (+100% to +150%)
Monster Pack Size: (+30% to +50%)20% more Monster Life
Monsters cannot be Shocked
30% increased Monster Damage
+35% Monster Lightning Resistance
Monsters deal 35% extra Damage as LightningTo the east,
it cannot be seen.
To the west,
it cannot be touched.
To the south,
it cannot be remembered.
And to the north,
it cannot be contained.Travel to this Map by using it in the Eternal Laboratory or a personal Map Device. Maps can only be used once.).
- Shaper's Orbs are sometimes awarded for completing a map; upgrades a map which allows "Shaped" maps to drop which are 5 tiers higher than their regular counterpart
- Cartographer's Sextants allow applying a special modifier to map, which applies its stats in a radius to other nearby maps. Those stats can be stacked, but a mod is used up after 5 uses 
- Unshaping Orbs can be used to unshape a map.
- Cartographer's Seals "uncomplete" a map.
Obtaining Shaper's Orbs
Players can acquire Shaper's Orbs by completing bonus objectives on specific maps spread around the Atlas. Their tier is equal to the tier of the map they dropped from minus 5, up to tier 10 orbs from tier 15 maps. Unique maps do not follow this rule, and award a specific tier orb specific to that map.
Spending Shaper's Orbs
Shaper's Orbs are used to upgrade lower tier maps whose layout or bosses a player may find accessible but whose drops and experience gains wouldn't otherwise be attractive. A Shaper's Orb grants +5 to the tier of those maps, i.e. increases the difficulty. Subsequently that increases the experience gained from slain monsters and item level of their drops. Note that Shaper's Orbs don't add Increased Item Rarity or Increased Item Quantity to that map. Unique maps can't be shaped.
Shaping a map can be undone using an Unshaping OrbUnshaping OrbStack Size: 10Downgrades a map on the AtlasRight click this item then left click a shaped map on the atlas to downgrade it. You can then earn the Shaper's Orb again to reshape another map afterwards. which is obtained by vendoring 20 Cartographer's ChiselsCartographer's ChiselStack Size: 20Acquisition
Level: 52Improves the quality of a mapRight click this item then left click a map to apply it. Has greater effect on lower rarity maps. The maximum quality is 20%.
Shift click to unstack. and 5 Orbs of RegretOrb of RegretStack Size: 40Acquisition
Level: 2Grants a passive skill refund pointRight click on this item to use it.
Shift click to unstack.. Once unshaped, players must redo the bonus objective previously completely on their Atlas in order to re-obtain the Shaper's Orb.
End of league transfer
The merge has the following rules:
- Maps or bonus objectives completed transferred to the parent league
- Cartographer's Sextant mods:
- They will be moved to the parent league; if the map already has a mod, the parent league mods will be kept
- Legacy modifiers continue to exist and work as long they have not been overridden
- Shaped maps will carry over if the equivalent level Shaper's Orb hasn't been used on parent league; if you already shaped a map of the tier the parent league takes preference
- Shaper's Orbs:
- Masterseek (August 13, 2016). "Path of Exile: Atlas of Worlds Interview. A Map of Condensed Hype.". 2p.com. Retrieved August 13, 2016.
- Chris (August 29, 2016). "Forum - Development Manifesto - Cumulative Atlas Completion Bonus - Path of Exile". Grinding Gear Games. Retrieved August 29, 2016.
- Negitivefrags (November 12, 2016). "[Question]What happens to the atlas and the sextant mods when essence gets transferred to standard?". Reddit. Retrieved January 25, 2017.