Here is the full list of all 40 Overlord: Fellowship of Evil achievements worth 1,000 gamerscore.
You defeated Clarisa.
You defeated Cedric.
You completed act 2.
You defeated the Void Gazer.
You summoned 500 minions.
You fully upgraded one minion colour.
You introduced the peasants to the zombies!
You completed act 1.
You bought half of one character's statue upgrades.
You killed 50 sheep
Your Browns killed 50 enemies.
You summoned 250 minions.
You crashed the banquet.
You completed the prologue.
You had 12 or more minions active at a single moment.
You gained 100% of the crowns in one quest.
Your Blues healed 500 times.
You knocked 20 enemies off the Netherdeep and into the Abyss!
You helped Ricket fix her forge.
You killed a "boss enemy" in a multiplayer game.
You defeated the Trolls.
You completed a quest in online multiplayer!
You completed the tutorial.
You summoned 50 minions.
SECRET
Obtain all weapons for all characters
Gain 100 crowns in-game through winning phases
With four players, each summon 4 minions at the same time.
Summon Green minions to assassinate 50 enemies
Kill 100 enemies with explosive red minions
Receive loot from minions 250 times
Complete/win 100 traversal states within the time limit
Buy all of the upgrades for all minion colours.
Buy all of one character's statue upgrades.
Buy all statue upgrades for all four characters.
Complete a quest in local co-op with at least 2 players present.
Finish a quest in 1st place in multiplayer