A List of Secret Achievements with descriptions.
Riders on the Swarm
Face a swarm effect with 4 open doors, cover the wide box and survive!
Open all 4 doors while fleeing. Hopefully one of the encounters is either a Goblin or a Plague Rat. The armor box should say x. Fight it and, as long as you cover the x armor box and survive, the achievement will unlock.
The Better Part of Valor
Descend with 4 open doors.
Open all 4 doors. Beat an encounter and explore again for more doors, effectively eating up time. Use Archer's Heroic Feat to speed it up even more. Feel free to take some deliberate consequences in the form of time, leaving 6-8 cards in the deck. Pretty easy if timed well.
Lose a game by taking damage from the Flames effect.
Have a hero 1 hp away from death before fighting the Fire Elemental. Choose said hero and lose. Achievement unlocked!
Circling the Drain
Lose a game by converting an item with health due to the Drain effect.
Might take some time. You need to have 0 potions and have a health item. Be 1 hp away from death before fighting the Wraith. Convert the health item to XP and game over man. But hey you got the achievement!