Duels in modern Yu-Gi-Oh! are often defined by the extra deck. The extra deck is a collection of up to 15 fusions, synchro, xyz, or link monsters. These monster cards all require specific summoning conditions to be met before they can be summoned to the field.
There are a number of extra deck monsters available for use in Yu-Gi-Oh! Master Duel, but the best ones are cards that can be splashed into as many decks as possible while still providing their user access to their powerful effects. The strength of an extra deck monster is greatly dependent on how good its effects are and how difficult it is to bring out.
10 Destiny HERO - Destroyer Phoenix Enforcer
Destiny HERO - Destroyer Phoenix Enforcer or D.P.E. for short, is a level 8 dark warrior-type fusion monster. This fusion monster must be fusion summoned using 1 "Destiny Hero" monster and a level 6 or higher "Hero" monster. This card can be fusion summoned using the spell card Fusion Destiny, which sends fusion materials from the hand or deck to the graveyard.
D.P.E.'s main effect allows it to destroy one card on both players' fields once per turn and if D.P.E. is destroyed by battle or card effect, the user can special summon one Destiny Hero monster from their graveyard during the standby phase of the next turn. D.P.E. can win players' games by slowly dwindling their opponent's resources down over time.
9 Knightmare Phoenix
Knightmare Phoenix is a link-2 fire attribute fiend-type monster. Knightmare Phoenix can be summoned by tributing any two monsters with different names. When this card is link summoned, the user can discard one card from their hand, then they can target one spell or trap card their opponent controls and destroy it.
If Knightmare Phoenix was co-linked when its effect was activated, the user can draw one card. Knightmare Phoenix also protects co-linked monsters from being destroyed by battle. This card grants practically every deck that runs it a reliable method of spell and trap removal.
8 Nightmare Unicorn
Nightmare Unicorn is a link-3 dark attribute fiend-type monster. It must be summoned by tributing at least 2 monsters with different names. When Unicorn is link summoned to the field, the user can discard 1 card in order to target and shuffle one card on their opponent's field into the deck.
If Unicorn is co-linked when this effect activates the user can draw one card. Unicorn is great for getting rid of monsters that can't be destroyed by battle or card effects and spell or trap cards that are causing trouble.
7 Baronne de Fleur
Baronne de Fleur is a level 10 wind attribute warrior-type synchro monster. On the surface, this card seems the like it isn't very splashable at all, since not every deck runs a tuner engine. This is true, but most decks in Yu-Gi-Oh! Master Duel run hand traps and many of the most popular hand traps in the game are tuner monsters.
Baronne de Fleur has two main effects. First, it can target one card on the field and destroy it once per turn. Its second effect can be used once while it is face-up on the field, it can negate the activation of any one card and destroy it.
6 Apollousa, Bow Of The Goddess
Apollousa, Bow of the Goddess is a link-4 wind attribute fairy-type monster. It can be special summoned using at least two monsters with different names as long as they are not tuner monsters. Each player can only control one Apollousa, Bow of the Goddess at a time.
Apollousa's effect is pretty simple. It gains attack points equal to 800x the number of monsters used for its link summon. It can then negate monster effects by reducing its attack points by 800 for each negate. This card is great for decks that can get a lot of monsters on the field. Just keep an eye out for cards like Nibiru and Lightning Storm.
5 Starving Venom Fusion Dragon
Starving Venom Fusion Dragon is a level 8 dark dragon-type fusion monster. It must be fusion summoned using 2 dark attribute monsters on the field that are not tokens. When Starving Venom is fusion summoned it can target one monster on the opponent's field that was special summoned and gain attack points equal to that monster's attack points until the end of the turn.
Starving Venom also has the effect to copy the name and effect of one monster on the opponent's field that is level 5 or higher and was special summoned to the field. If Starving Venom is destroyed, the user can destroy all special summoned cards on the opponent's field. Starving Venom players will get the most out of this card if they pair it with cards like Super Polymerization and Fallen of Albaz.
4 Mekk-Knight Crusadia Avramax
Mekk-Knight Crusadia Avramax is a link-4 light cyberse-type link monster. It requires at least 2 monster special summoned from the extra deck to be used as material for its link summon. Avramax cannot be targeted by the opponent's card effects. It also prevents the opponent's monsters from attacking any other monster but itself.
Avramax also has the effect to gain attack points equal to the opponent's monster's attack points once per battle, during damage calculation. If that still wasn't enough, Avramax shuffles 1 of the opponent's cards on the field into the deck when it's sent to the graveyard by an opponent's card effect.
3 Predaplant Verte Anaconda
Predaplant Verte Anaconda is a link-2 dark plant-type monster. It requires two effect monsters to be used as material for its link summon. Anaconda has two effects. Its first effect allows it to change 1 monster's attribute on the field to the dark attribute. Its second effect allows it to send one normal or quick-play spell card from the deck to the graveyard in order to copy its effect.
This allows Anaconda to search out and activate some of the most powerful fusion spell cards in Yu-Gi-Oh! Master Duel. Cards like Super Polymerization, Branded Fusion, and Fusion Destiny. Anaconda makes pretty much every other fusion deck that fusion summons from the deck splashable in other decks as an engine.
2 I:P Masquerena
I:P Masquerena is a link-2 dark cyberse-type link monster. It requires two non-link monsters to be used as material for its link summon. Masquerena's effect comes in 2 parts. The first effect allows the user to tribute I:P Masquerena and any other monsters on their side of the field during the opponent's turn in order to link summon a monster to the field.
The second effect is applied passively to the monster summoned using I:P Masquerena as material. That link-summoned monster can not be destroyed by the opponent's card effects. I:P Masquerena makes so many other link monsters better through both of its effects. Cards like Nightmare Unicorn can use their effects during the opponent's turn as a form of disruption and cards like Avramax become almost impossible to get rid of for most decks.
1 Accesscode Talker
Accesscode Talker is a link-4 dark cyberse-type link monster. It requires at least 2 effect monsters to be tributed for its summon. Accesscode is one of the strongest extra deck monsters ever released, and it can be used to help many decks close out games thanks to its fantastic effects.
Accesscode gains 1000 attack points x the link rating of one card used for its link summon. This means Accesscode will oftentimes be hitting the field with 4300 or 5300 attack points. Its second effect allows it to destroy cards on the field by banishing link monsters from the graveyard with different attributes. Also, the opponent cannot respond to Accesscode Talker's effects when they are activated.
Yu-Gi-Oh! Mater Duel is available on PC, PlayStation 4, Xbox One, PlayStation 5, Xbox Series X/S, Nintendo Switch, Android, and iOS.