Fusion is a new card modification added in 1.25.3

Fusion allows you to combine two identical Level 5 Cards to gain an additional ability for the card. For all Combo Cards the Fusion bonus is to gain an extra combo point when played. Combining two Fused Combo Cards will result in granting 3 combo points. All Final Form Cards gain some additional ability from a list, which is activated by paying Combo Points when played. Fusion bonuses are from a pool of abilities and can be viewed for any card in the Upgrade screen when a card is selected (far right form) or on the Card Info page from the main screen.

Though this is not very significant, it is important to note that as of 1.27.3, the Siphon and Absorb fusions are unusable at this time, due to the game crashing. Hopefully, Chinzilla will fix this in their next update.

Tip: The most common strategy is to compose a deck full of Bonus Orb, Crushing Blow, and Critial Strike.

Fusion Bonuses 

Bonus Cost Description Cards with bonus
Orb Orb Free Gain an extra orb when combo'd ALL Combo Cards
Pierce Pierce 3 Ignore half the enemy card defense Anaconda, Assassin, Bomb, Catapult, Cobra, Dark Hero, Excalibur, Farmer, Giant Sword, Gladiator, Goblin, Laser Sword, Megalodon, Morning Star, Night Dragon, Pirate Zombie, Poison Dagger, Poseidon, Raging Bird, Saberchilla, Scholar, Scythe, Shadow Prelate, Sir Lancelot, Snakes on a Jet, The Badger, Triceratops, Unicorn, Vampire Hunter, Vulture, Wasp, Wyvern
Crushing Blow Crushing Blow 5 Enemy card defense is completely ignored

Alphatron, Ancient Dragon, Barrel Gorilla, Black Friday, Cerberus, Dante (Card), Dragon Cannon, Frankenstein, Hammertron, Hot Pepper, Ifrit, Jaeger, Leopold (Card), Lion, Mecha Bear, Mudslide, Otto (Card), Panda, Polar Bear, Punchy, Smog Horror, Smug, Spider Mine, Thor's Hammer, Troll, Turkeyzilla

Block Block 3 Increase your card's defense by half Arch Angel, Armored Dragon, Brontosaurus, Car, Chinchilla Knight, Dwarf, Fairy Godmother, Gargoyle, Gnome, Grabbers, Green Knight, Guardian Angel, Ice Golem, Indiana Ford, King, Kraken, Legendary Hero, Lochness Monster, Metal Golem, Paladin, Rock Slide, Steel Hero, Submarine, Wonderful Chin,Wooden Golem
Protection Protection 5 Double your card's defense Adventure Kid, Astronaut, Conan, Crazy Cat Person, Cyberwolf, Cyborg, Great Wall, Gummy Bear, Hammer Time, Healthy Hero, Heavenly Dog, Heavens, Kate (Card), King Kyle (Card), Transformobot, Rainbow Girl, Scarecrow, Space Guardian, Windmill of Doom,
Reflect Reflect 3 Reflect half the damage you receive this turn Angel of Wisdom, Arch Mage, Baseball Bat (highest reflect damage), Battle Mage, Blacksmith, Blue Knight, Caveman, Death's Horse, Demonite, Dire Wolf, Dragon Rider, Hydra, Flame Sword, Lil' Android, Mutant Man, Ninchilla, Pegasus, Pestilence's Horse, Red Knight, Seahorse, Shapeshifter, Sledge Hammer, Super Bat, Thor, Treant, Voidwalker, War God
Counter Attack Counter Attack 5 Reflect all the damage you receive this turn Air Elemental, Alien, Aeromancer, Bill, Burgerlar, Half-Life, Harpy, Hydrabot, Mummy, Phil (Card) (highest counter damage), Raccoon Guardian, Slithers (Card) (also highest counter damage), The Grater, Turkey Zombie, Ultimate Soldier, Werebear
Siphon Siphon 3 Heal yourself for half the damage you deal this turn Aviator Chin, Black Hole, Bone Sword, Chinzombie, Clown, Dark Angel, Dark Matter, Dracula, Elf Wizard, Fire Sprite, Firefox, Flood, Headless Rider, Hydromancer, Killer Plant, Learning Rainbow, Maelstrom (Diamond Card), Mama Brain, Nightshade, Ocean Defender, Phoenix, Skeletal Dragon, Soul Stealer, The Snake Lord, The World Tree, Undead Knight, Vampchilla, Void Fiend, Water Sprite, Werevamp, Zombiesaur
Absorb Absorb 5 Heal yourself for all the damage you deal this turn

Avery (Card), Bridget (Card), Cyclone, Dark Alchemist (Card), Death Ray, Elf Baker, Graham (Card), Harvest, Hey Cat, Ivana (Card), Jack the Dog, Jim (Card), Lucy (Card), Magnet Dude, Mind Slug, Puff Hero, Sandman, Spirit Wolf, Titan of Death, Xanthar (Card), Xenomorph, Young Wolf, Zombie Bear

Amplify Amplify 3 Increase your card's attack by half Arcane Golem, Bionic Chinchilla, Bomb, Cavalry, Centaur, Chinzilla, Club, Death Metal, Double Rainbow, Fireball, Fire Golem, Geomancer, Gorezilla, Jet Fighter, Lava, Meteor, Muse, Puff, Pyromancer, Raging Bird, Super Nova (highest), The Incinerator, The Reaper, Ultra Dude, USB of 999 Truths, Void Horror, Wildfire, Wolfpack
CriticalStrike Critical Strike 5 Double your card's attack

Anna (Card), Ariel (Card), Bear Magus, Charles (Card), Chloe (Card), D-3000, Blue Bull, Dark Elf, Fireworks, Gelmancer, General Viper, Gummy Worm, Hubert (Card), Janice (Card), Laser, Leon (Card), Meowmon, Monster Slayer, Ouroboros, Pastry Cat, Plasma Sword, Poison Oak, Pup, Robot Devil, Sabertooth Cat, Sniper (Highest), Windmaster Eng, Ziz, Snowflake

Weaken Weaken 3 Enemy card attack is halved Care Chilla, Cleric, Cthulhu, Devil, Druid, Dryad, Dust Bath, Earthquake, Friendly Dino, Ghost, Imp, Leprechaun, Medusa, Mermaid, Mr. Ted, Necromancer, Nightmare, Poison Dragon, Professor Y, Pterodactyl, Skeletal Mage, Steam, The Magician, Void Shaman, Warlock, War's Horse, Witch, Witch Doctor, Wiz's Apprentice
Curse Curse 5 Enemy card attack is removed Alien Prophet, Alien Sorceress, Ancient Ark, Angel of Wrath, Albert (Card), Berserker, Black Cat, Black Death, Blood Angel, Blood Imp, Blood Mage, Blood Moon, Bloody Mary, Bluebeard, Bone Xylophone, Book Burner, Brian (Card), Broomstick, Candy Witch, Carver of Doom, Changeling, Charon, Cheshire Cat, Cinderella, Cleopatra, Clown Toxin, Commander of Sin, Computer Bug, Conch Shell of Doom, Court Jester, Crimson Witch, Crying Angel, Cursed Apple, Cursed Flute, Cursed Garden, Cursed Monkey Paw, Cyborg Alien Mutant, Cymbal Monkey, Dark Swan, Davy Jones, Death Adder Shadow, Demon Queen, Demonic Fighter, Doll, Dr. Savage, Dragon Incarnate, Emok, Egyptian Tomb, Einstein, Elmoira (Card), Evil Queen, Evil Vizier, Firefly, Flux Inventor, Flying Goblin, Flying Monkey, Flying Trooper, Frog Prince, Furies, Gale (Card), Genie, Golden Harp, Golden Scarab, Ghost Army, Green Eyed Monster, Grouchy Monster, Groundhog, Gus (Card), Harmony (Card), Henry (Card), Ice Age, Ice Imp, Ice King, Igor, Inner Demons, Jack-o'-lantern, Jasper (Card), King Dodo (Card), Koala, Lich, Luna (Card), Magicianbot, Magicod, Malevolent, Master Alchemist, Master Dueler, Mr. Beetleson, Monster & Beauty, Morgana, Multiverse, Mumbling Kid, Naga Queen, Nessa (Card), Nosferatu Duck, Orpheus, Pandora's Box, Paradox, PB & Jelly Time, Philosopher's Stone, Pinch, Pink Skull, Pinocchio, Pirate Booty, Plant Clones, Possession, Precious Ring, Precious Ring Bearer, Pumpkin Prince, Radioactive Avenger, Rapunzel, Ravenica, Ring Lord, Rip Van Winkle, Road Sprinter, Rumpelstiltskin, Sand Raider, Sandstorm, Sea Witch, Serpent Cultist, Sewer Mutants, Shadow People, Shifting Mutant, Shoggoth, Siren, Skeletal Horse, Sleeping Princess, Sleepwalker, Snake Oil Salesman, Soul Harvester, Space Zombie, Speed Demon, Sphinx, Spork of Doom, Swamp Monster, The Bone King, The Boy Wizard, The Cursed Chosen, The Ring Courier, Three Wise Monkeys, Tomb Keeper, Trapper-1, Turtle Sorcerer, Undead Bride, Vamp Crunch, Vera (Card), Vengeful Spirit, Video Game, Villain Groupies, Vlad (Card), Void Beast, Voodoo Doll, White Rabbit, Wight, Witch's Brew, Wizard Headmaster, Wizard Staff, Wraith, Youthful Sorceress, Zombie Fungus, Zombie Romeo