Nightmare Mode

Nightmare Mode is an upcoming mode for Dungeons and Campaign stages. The release date as of this writing (6/12/2015) is still not official, but it was accidentally released to those on the web client and was playable for those who did not refresh.
Nothing is finalized yet, but on cursory play-throughs, certain enemies were found to have different passive abilities as noted here:
Berserker - Unit does significantly more damage (due to raised attack) at low HP
Sharpeye - Unit Hit % and Crit % are doubled
Regenerator - Unit heals up to 500 HP per turn
Reflect - Reflects certain ranged attacks, including Wizard Fireballs and Dragoons Gaia Break. (still needs to be fully verified)
Anti-Ranged - Reduced damage from projectiles
Anti-Magic - Reduced damage from magic
Chosen - Unit has double Attack, Skill, and Magic, and triple Defense
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The rewards for the dungeons were not active at the time, but Crowns and coins were still given out.
Wizard Tower - Same coins and crowns as Hard Mode
Crypt Warrens - 
Frosthelm - 500 coins and 5 crowns for 1-3. 750 coins and 10 crowns for 4-6, 1000 coins and 15 crowns for 7-9. 1250 coins and 15 crowns for 10-12
Temple of the Bear God - 1-2 crown per stage, coins not verified.
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This mode also introduces a new enemy - the Banshee. The Banshee looks like the Fire Captains from the Flamemancer dungeon but is white instead of red. The Banshee has different abilities based on which stage you encounter him in.