Atlantis may refer to one of two things:  Season 2, or the monthly summons related to Season 2, also known by the same name.

Provinces, Stages, and Coins

Season 2 was released in August of 2018.  The completion of a stage for the first time will award 3 Atlantis Coins
Atlantis Coin
when completed on Normal, and another 5 coins for Hard completion (which is only available after first completing it on Normal).  Subsequent completions continue to award loot, but only the first successful completion on each difficulty awards coins.  The total number of coins that you have acquired from any particular Province/Stage, along with the total number of coins available, is shown when looking at the Season 2 map.

Season 2 is also known for having Special Stages.  There are currently 4 kinds of Special Stages, including Poison Mist, Magic Night, Dense Fog, and Underwater.  These Special Stages have their own Mission associated with them and make it somewhat more difficult to complete than a traditional stage (for instance, the Underwater stages penalize the use of Fire heroes).

Atlantis Summon

Atlantis Summon
The Atlantis Summon is fairly unique in that it is the only way to acquire previous Heroes of the Month.  The summons becomes available on the first Friday of the month and continues for 60 hours with the chance of awarding Season 2 heroes that are each generally considered superior to the Season 1 heroes available through the Training Camp.

The Atlantis Summon generally costs 350 gems per pull, or 3000 gems for a 10-pull.  There is also a built in system to accept 100 Atlantis Coins for a single pull, as well.

Each pull also fills a bonus chest that contains ascension materials.  For every 10 heroes summoned via the Atlantis Summon one bonus chest is awarded.

Featured HotM

Each month there are two 5* Atlantis heroes that are featured heroes, which improves the chances that you will pull those particular heroes.  There are also two previous Heroes of the Month that are fairly randomly chosen by SGG (i.e. difficult to predict) and possible to pull should lady luck be on your side. Some patterns can make certain heroes more likely, see Featured Hero History here.
Note that these previous HotM are considered a regular Atlantis pull, as such it is possible (though highly unlikely) for one to pull both a previous HotM along with the current HotM with just one pull.

Notable Atlantis Heroes

4* 5*
Proteus Ariel
Wilbur Poseidon

Atlantis Hero Families

Each Atlantis hero belongs to a particular family and will receive a small buff when they are placed on a team with another member of their family.  The three Atlantis families are:
Season 2 Family Chart

(Image thanks to Razor at TitanMafia )

Atlantis Rising

Atlantis Rises
Once Season 2 was completed SGG amplified the experience surrounding the Atlantis Summon by releasing Atlantis Rising.  The timing coincides with the availability of the summons (first Friday of the Month and runs for 60 hours).  It also:
  1. Reduces the energy cost of Season 2 stages
  2. Increases the loot reward for all Season 2 stages

It originally was the fourth Thursday of the month, but was changed due to the release of Season 3 in February of 2020.

Popular stages to run

Stage Reason
6-9 or 15-9 Recruits
4-9 Rugged Clothes
1-2 or 1-3 Adventurer's Kit
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