Here is the full list of all 24 Reignfall achievements.
Achieve victory on easy difficulty
Defeat wave 30 on easy difficulty
Achieve victory on medium difficulty
Defeat wave 30 on medium difficulty
Achieve victory on hard difficulty
Defeat wave 30 on hard difficulty
Achieve victory on brutal difficulty
Defeat wave 30 on brutal difficulty
Achieve victory on any difficulty above easy with no buildings lost
Achieve victory on any difficulty above easy with less than 50 troops lost
Achieve victory on hard or brutal difficulty in under 2 hours
Destroy the mysterious cave
Kill 1,000 enemies
Kill 5,000 enemies
Kill 10,000 enemies
Kill 50,000 enemies
Kill 100,000 enemies
Reach 100 kills with a unit
Reach 300 kills with a unit
Reach 500 kills with a unit
Achieve victory on hard or brutal difficulty without recruiting any heroes
Achieve victory on hard or brutal difficulty without building any towers
Achieve victory on hard or brutal difficulty with no Lord deaths
Achieve victory on hard or brutal difficulty without any starting items