Here is the full list of all 32 Hellbound achievements.
Find a secret area
Find the super secret area in The Heroes' Cemetery
Find the "You got stuck" message in Flying Souls
Find the developers message in The Core
Kill 100% of enemies in a campaign level
Pickup 100% of items in a campaign level
Find 100% of secrets in a camapain level
Complete a campaign level in 300%
Complete all campaign levels in 300%
Beat campaign level 1
Beat a campaign PAR time
Finish the campaign
Finish the campaign in Old School difficulty
Finish the campaign in HELLMARE! difficulty
Play (and die) in a survival arena
Get to the final wave in a survival arena
Survive more than 15 minutes in a survival arena
Pick up the Rocket Launcher
Pick up a Soul Orb
Die by lava
Die by an Exploding Pumpkin
Die by a Hell Knight Guard
Kill 3 enemies (or more) with a single shot of the Triple Shotgun
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 a Pygon using the Head Crusher
Kill an enemy and drop its body into the lava
Kill 30 enemies during Hell Damage
Kill Ferlord
Kill a total of 1000 enemies
Press the interact button and miss 12 times in a row
Check the entire credits