Creepy Treat Cards can be earned as rewards after Combat, earned as rewards for completing certain Side Quests, and bought from Shady.
Here is the list of cards with their abilities and locations -
Sugar Droppings - Single full heal: In a coffin in the Lower Bayou in the past. Use the Pterodactyl flap to get past a pile of leaves blocking a path to the right of the alligator pit.
Fudge Crystals - Splash damage on all attacks: In a coffin in the Lower Bayou in the past. Use the Pterodactyl flap to clear a pile of leaves blocking the entrance to a fenced area near a house not far from Shady's Secret Shop.
Nubbins - 2x attack damage for next 3 attacks: In a coffin in the Lower Bayou in the past. Use the Pterodactyl flap to get past a pile of leaves blocking a path near Shady's Secret Shop. Defeat the Savage Snail blocking the coffin.
Bubble Barf - Chosen Hero ignored by all enemies: Complete Getting Duped quest.
Suck 'ems - Chosen Hero takes 2 turns: Bought from Shady's Secret Shop Costs 175 candy.
Froot Flakes - Mute a minion's magic attacks: Trick-or-Treating in Lower Bayou.
Ghost Munch - A minion's attacks damage itself: Trick-or-Treating in Lower Bayou.
Goozers - An enemy's attacks heals rather than damage: Bought from Shady's Secret Shop Costs 225 candy.
Splorts - Amazing attacks are much easier: Trick-or-Treating in Lower Bayou.
Flippity Flaps - Fully heals your heroes after you win: Witches' Bog, Defeat the Depraved Mole. (Unmissable)
Frostbites - Max out 1 hero's special meter: Defeat Kronoculus. (Unmissable)
Carameals - Cuts a minion's health in half on each turn: Bought from Shady's Secret Shop Costs 125 candy.
Snickets - Double candy reward: Bought from Shady's Secret Shop Costs 125 candy.
Bone-Bones - Stun a minion for 3 turns: Bought from Shady's Secret Shop Costs 175 candy.
Tin Foil Chews - Heals a hero by 25% for 3 turns: Bought from Shady's Secret Shop Costs 225 candy.
Jumpin' Beans - Strong Defense against your "weak against" actor type: Defeat one of the drones in the Old Auburn Pines neighborhood, will be a after battle reward.
Tongue Punchers - Double Experience Reward: Defeat the Dental Drone across the street from the park in the Old Auburn Pines neighborhood in the future.
Candy Teeth - Weaken all minion attacks for remainder of fight: In a coffin in New Repugia, Downtown in the future.
Pumpkin Slough - Undo last turn of damage by enemies: Defeat a Grubbin at one of the trick-or-treat houses in the Old Auburn Pines neighborhood during Sadie's Speakeasy Deliveries. The house you want to Trick-or-Treat is across the street from the park.
Toffee Taffy - Cancel all enemy defuffs on players: Bought from Shady's Secret Shop Costs 475 candy. (Level 4 Unlock)
Happy Heads - Minion attacks are half as strong for 3 turns: In a coffin in New Repugia, East Side in the future.
Rice Creepies - Next minion that attacks a chosen player is automatically defeated: Defeat the Savage Snail in the Old Auburn Pines neighborhood. In the backyard of the house next door to Everett & Lucy.
Schafer's Wafers - 50% max HP heal to all: In a coffin in New Repugia, Downtown in the future.
Sour Dumps - Max out all 3 heroes' special meters: Bought from Shady's Secret Shop Costs 375 candy. (Level 4 unlock)
Pie Cake - All minions drain 25% health per turn: Bought from Shady's Secret Shop. (Level 4 unlock)
Headless Horsemints - Next 3 attacks by any hero are automatically Amazing: Bought from Shady's Secret Shop. (Level 5 unlock)
Mystery Lumps - Next 3 defends are an automatic block: Bought from Shady's Secret Shop. (Level 6 unlock)
Delecta-Bulls - Stan all enemies for 1 turn: Bought from Shady's Secret Shop. (Level 6 unlock)
Licky Plops - Obliterate 1 minion: Trade a spare Rice Creepies card to Margot NPC in Old Auburn Pines.
Sugar Uncles - Hero strong against all enemies, All minions weak against hero: In a coffin in the Lower Bayou in the past. Use the Pterodactyl flap to get past a pile of leaves blocking a path to the right of the alligator pit.
Dippin'Stix - All heroes contribute to chosen hero's special meter: A random monster drop, Easily obtainable.
Nougat Nuggets - All heroes attack at once: In a coffin on the rooftops of the French Quarter in the past.
King Size Fun Size Bar - All minions debuffs boomerang back to themselves: In a coffin in Dr. White's Inner Sanctum in the future.
Peepers - Hero's attack does 3x damage, returns 1x damage to self: Defeat Principal BoJimm during Escape! (Unmissable)
Jelly Beaks - Reset Cooldowns on the other cards in your active deck: A random monster drop, Easily obtainable.
Moldy Mounds - All attacks cause extra fire damage for remainder of the battle: A random monster drop, Easily obtainable.
Garlic Crunch - 2x battle speed: Bought from Shady's Secret Shop. (Level 8 unlock)
Fling-a-lings - All heroes are strong against all enemy types: In a coffin in Dr. White's Inner Sanctum in the future.
Smoochies - Max all special meters after win: Mole mini-boss in Dental compound has one at 100% drop rate.
Cherry Klots - Auto-resurrect next defeated hero with 50% health for current battle: Bought from Shady's Secret Shop. (Level 8 unlock)
Morons - All remaining attacks by hero are Amazing: Bought from Shady's Secret Shop. (Level 7 unlock)
Tongue Ties - Perfect Counter on next 3 defends: Trade a spare Smoochies card to Jho NPC in Dr. White's Inner Sanctum.
Pampla-Mooses - Grants additional 100 candy after win: Bought from Shady's Secret Shop. (Level 7 unlock)
Cthulhu Chew - All heroes are invulnerable for next defensive round: Bought from Shady's Secret Shop. (Level 9 unlock)
Peanut Butter Bomb - Obliterate all minions: Bought from Shady's Secret Shop. (Level 9 unlock)
The video below shows all chest/coffin related card, credit goes to darkavenger786 for the video.
And credit goes to VidGamiac for uploading the video.
This guide is far from perfect, If you could give me any details on the cards please let me know.
Negative voters please let me know why so i can make changes.