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