Here is the full list of all 20 Netherguild achievements.
Reach level 3 with any character
Have your guild members suffer 200 damage overall
Returned from your first expedition.
Deal 10 or more total damage in a single attack (includes AoE, excludes status)
Reach level 2 with any character
Reached the Mist City campsite
Defeat "Pig", the Purple Bandit Leader
Choose to release Pig the Purple Bandit Leader
Choose to capture Pig the Purple Bandit Leader
Choose to execute Pig the Purple Bandit Leader
Reached the Mist City Streets campsite
Reach level 4 with any character
Deal over 1,000 damage in total
Amass 500 Crowns or more
Deal 20 or more total damage in a single attack (includes AoE, excludes status)
Block an enemy attack successfully
Restore all four characters to maximum health by eating next to a bonfire.
Push an enemy into the next floor without killing them.
Piece through units with all the targeted tiles of the "Icicle Piercer" attack.
Hit a unit with every targeted tile of the "Rain of Arrows" attack.