Here is the full list of all 32 Hellbound achievements.
Find the "You got stuck" message in Flying Souls
Find the developers message in The Core
Find the super secret area in The Heroes' Cemetery
Play (and die) in a survival arena
Check the entire credits
Finish the campaign
Finish the campaign in Old School difficulty
Kill Ferlord
Kill a Pygon using the Head Crusher
Pick up the Rocket Launcher
Die by an Exploding Pumpkin
Pick up a Soul Orb
Kill 10 enemies with the Indolora without releasing the trigger
Find 100% of secrets in a camapain level
Complete a campaign level in 300%
Pickup 100% of items in a campaign level
Kill 3 enemies (or more) with a single shot of the Triple Shotgun
Kill a total of 1000 enemies
Press the interact button and miss 12 times in a row
Beat a campaign PAR time
Beat campaign level 1
Kill 100% of enemies in a campaign level
Die by a Hell Knight Guard
Kill an enemy and drop its body into the lava
Find a secret area
Die by lava
Complete all campaign levels in 300%
Finish the campaign in HELLMARE! difficulty
Get to the final wave in a survival arena
Survive more than 15 minutes in a survival arena
Kill 5 enemies (or more) with a single rocket or grenade of the Rocket Launcher
Kill 30 enemies during Hell Damage