Here is the full list of all 18 Soul Nomad & the World Eaters achievements.
Refuse Gig's help on the 4th map
Summon a second squad
Receive Gig from Layna
Train with Danette for the 2,327th time
Start down the Demon Path by slaughtering your village
Reach a normal path ending
Accept Gig's MAX Gigify against Feinne and get the Bad End on the 4th map
Reach the Demon Path's Bad End
Reach the Demon Path's Good End
Spend 10,000 Gig Points
Reach level 100 on a character
Start your second cycle
Unlock Asagi
Unlock Lord Median
Unlock Lujei
Find the Overlord
Unlock all rooms
Defeat a Level 100 ultimate hero