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