Here is the full list of all 38 Halloween Forever achievements.
You bested Chainsaw Megamaniac! There's no chainsaw maniac tougher than this one.
You bested Big Bat! Have you ever seen a bat so big?
You bested Green Hydra! Two heads are better than none, but that won't slow us down.
You bested Red Devil! Yow, it's hot in here!
You bested Eyeskuller! Eye spy with my little eye something that's about to explode!
You bested Necromancer! Back to ghoul school with you, fool!
You bested Manleech! Say your lamprey-ers!
You bested Frog King! I toad you so!
You bested Blue Devil! Necessity is the moth of invention.
You bested Necroking! The return of Necromancer! How the mighty have fallen.
You rescued Butternut Bro! Your gourd from another lord.
You won as Butternut Bro! Way to squash the opposition.
You rescued Ms. Witch! She has endless cats!
You won as Ms. Witch! Hope you had a feline time.
You rescued A Skeleton! A bony vertebra buddy.
You won as A Skeleton! I knew you had it in you (I can see through your ribgaps).
Make Chainsaw Megamaniac leave his chainsaw behind when he dies.
Destroy five hanging skeletons before they hit the ground.
Knock five spiders off the wall.
Wake up 6 sleepy frogs.
Never harm little bats! Ever!
Destroy a breakable wall.
Find all six hidden runes for a secret ending!
Beat a world without taking a hit.
Beat the game with 1 HP Mode enabled! HARDCORE!!!
Get grabbed by Blue Devil three times.
Watch Manleech vomit 10 times. Yucky!
Trigger spike traps 101 times.
Set loose 666 cats as Ms. Witch.
Run away from a crow for a very long time.
You won as Pumpkin Man! Get pumped!