Here is the full list of all 31 Realm of the Mad God achievements.
More than 1000 party member level ups with a single character
Completed over 65 Class Quests
Completed over 52 Class Quests
Completed over 39 Class Quests
Completed over 26 Class Quests
Defeated more than 1000 gods with a single character
Achieved fame of 1000 or more
Unlocked Mystic Class
More than 1000 quests completed with a single character
More than 4 million tiles uncovered with a single character
Dealt Killing blow to Oryx
Achieved fame of 500
Defeated more than 100 gods with a single character
More than 100 party member level ups with a single character
More than 1 million tiles uncovered with a single character
Completed over 13 Class Quests
Unlocked Trickster Class
Unlocked Sorcerer Class
Unlocked Huntress Class
Unlocked Rogue Class
Unlocked Paladin Class
Unlocked Knight Class
Completed every dungeon type with a single character
Unlocked Assassin Class
Unlocked Necromancer Class
Unlocked Priest Class
Unlocked Warrior Class
Reached Level 10
Reached Level 20
First in a long line of heroes
Unlocked Archer Class