The War of Heroes expansion was the third to be released by playmage. It came out on June 27, 2012, and it's main feature was the release of the CCG by the same name. This was release where the hero slots became unlockable by coupons.

Here's the full list of features of the War of Heroes expansion from the Announcements and Patches page on Kongregate:

A new supreme evil being by the name of “Summoner” has appeared. He has summoned minions of evil to attack innocent civilian in each galaxy. Since he hides in his cavern, our heroes must master the art of ground combat and defeat him on foot.

  • New “War of Heroes” gameplay (CCG + TD)
    • Occurs in random encounter chance (50%) when visiting someone’s planet for chapter 3+ players. Difficulty ordered by planet.
    • New “Summoner” raid boss for chapter 4+ players in the raid team creation menu. Everyone gets 5 FREE drawing each day after defeating the the raid boss. The boss summons large amount of mobs so 3-4 player raid team are recommended.
    • In the beginning, all players are expected to only be able to defeat pirates in the first 2 planets and easy mode of the boss. It is expected to take couple months for high level players to be able to defeat 4th/5th planet and the hard boss difficulty.
    • Includes new main character equipments and skill card crafting system.
    • Everyone will receive a FREE gift package with 4 equipments and 4 skill cards!
    • Crystals to craft skill cards drops from both random encounters and as raid boss prizes.
    • Play the tutorial and see help menu section for more how it works.
    • The new gameplay is still not tweaked perfectly yet, so feel free to submit your feedback but remember to be nice.
    • The new gameplay does takes more time (fun time-killer), but IS completely OPTIONAL. It has following minor benefits:
      • Random encounter can reward good experience once you are able to defeat mobs of higher level planets.
      • Main character equipment (dropped by raid boss) have a set bonus that slightly increases ship attack.
      • Higher difficulty of raid boss can reward good amount of resources.
      • Raid boss also have a small chance (10%) of dropping a pillar crystal that repairs 5M damage to your pillar and rewards 1000 coupons when used.
  • Added headgear hero equipments
    • Head gears are rarer level 40+ only equipments. This means they will not drop as loots in the first 7 chapters.
    • They can be found in mall chests and auction tab, but at rarer rate than other equipments.
  • Maximum hero level now increased to 65
  • Viewable profiles
    • You can now view other player’s profile and avatar by clicking on their picture from visit/attack context menu.
    • You can also go the “War of Heroes” tab to view the new equitable avatar that shows up in hero battles.
  • New Auction Tab in the mall
    • The auction items now have their own tab.
    • 6 items will be up for auction at the same time.
    • Gold items and specific skill books might be auctioned in addition to equipments.

Various Improvements

  • Hero slots can now be unlocked with coupons.
  • Last chat tab is now remembered and auto set when returning to the game.
  • Doubled maximum length of mail.
  • You can now mouse-over the hero exp bars to see the actual current level exp and required exp.
  • You can now mouse-over the percentage loss in the ship battle report to see actual army score losses.
  • Once you accept to join a raid team, it now automatically opens the raid team dialog (which you can choose to close).

Also of interest, but unknown if was originally intended to be a part of War of Heroes expansion was the addition of the WoH Arena on July 24, 2012. Here are the notes on that update from the Announcements and Patches page on Kongregate.

PvP War of Heroes Tournaments!

  • Tournaments starts at 22:00 server time on Mondays and ends on 22:00 server time Sundays.
  • There are 3 separate tournaments. 1v1, 2v2, and 3v3.
  • In each tournament, awesome prizes will be rewarded to the top 100 ranked players. Ranks and prizes are showed in the Arena.
  • Players are ranked based on a standard tournament “score” system. Everyone starts with a score of 1000, and will earn score when they win and lose score when they lose.
  • You earn a higher amount of score when you defeat someone with relatively higher score than you or your team. You earn less when you fight players or teams with lower score or if you fight same player/team multiple times within a set amount of time.
  • Each tournament has a minimum number of battles required to qualify for rewards. Even if your score is in the top 100, you may not win a prize if you didn’t fight the minimum number of battles.
  • Prizes are mailed to the winners when the tournament ends. Anyone found abusing/cheating in the tournament will have their account suspended or banned.
  • Once you create your team and click start, the game will try to match you against another player/team that also have started the matching process with similar score, and will wait up to 1 minute for similar score player/team to join of one doesn’t exist. After one minute, it will try to match you against any player/team that is also looking for a match.
  • Each battle in the tournament costs 1 action point for all participants.