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"Machine King" is a series of monsters who, for the most part, gain ATK when there are other Machine-Type monsters on the field. The lores of some Machine-Type cards mention serving the "Machine King". However, it is unknown which model of the "Machine King" they serve (most likely "Machine King", since it was generally the only card of that name out at the time of their release, but this could generally mean any of them).
Machine King Deck
A Machine King Deck focuses on the titular cards and swarming the field with Machine-Type monsters. However, this is not limited to your own monsters, you can also swarm your opponent's field and then use "DNA Surgery" to change them all into Machines.
This deck is essentially a Billy Beatdown Deck, as "Machine King" and its counterparts all focus on increasing its own attack through your own field presence. Commonly used with it is "DNA Surgery", so that your opponent's monsters are also changed to machines.
Because "Machine King" and "DNA Surgery" can only benefit/affect face-up monsters, "Light of Intervention" will immensely help, as well as to get rid of annoying effects. You should have three copies of DNA Surgery and perhaps one or two "Light of Intervention" to speed this up, as it will allow even "Machine King Prototype" to gain a substantial ATK boost. "United We Stand" is also useful, as well as "Fairy of the Spring" to return "United". "Big Bang Shot" and "Fairy Meteor Crush" are likewise good choices for this Deck, since your opponent will more than likely play monsters in Defense Position hoping to prevent potential damage. Machine support cards such as "Limiter Removal" work well in this Deck too. Although using "Limiter Removal" is somewhat reckless, doubling the ATK of all your Machine-Type Monsters for one turn could be just what you need to win the Duel. However, the real power of this Deck lies in your ability to swarm the field with monsters, including your opponent's field if you have "DNA Surgery", because more monsters means more ATK for "Machine King". So be sure to include cards that can swarm, such as "Phantom Skyblaster" and "Grinder Golem", for they each create Tokens to power up your "Machine King" monsters. "Creature Swap" makes a great addition if you decide to use either monster, allowing you to give your weaker Token monster to your opponent while they give you one of their actual monsters. Also, having many Tokens gives you free Tribute for activating "Nightmare Archfiends". Giving your opponent 3 high ATK monsters might seem like a bad idea at first, but once you use the Trap card "Remove Brainwashing", all those high ATK monsters will become yours assuming you have unoccupied Monster Zones, if not your opponent will take 800 points of damage for each Token destroyed, which is favorable result anyway. Not to mention that the same works for "Grinder Golem" which will switch to your control as well when "Remove Brainwashing" is on the field giving you a whopping 3000 ATK monster in addition to your "Machine Kings".
Ironically enough, because this Deck uses "DNA Surgery" to change monsters to Machine-type, Machines of over 500 ATK are not really needed unless they have a substantial or effect, such as "Cyber Dragon". "Gilasaurus" and "Inferno Reckless Summon" can put more monsters on your opponent's field. While this is normally avoided, each monster will grant a "Machine King" an additional ATK boost. However, Token creation cards such as "Scapegoat" can fill your monster zone with Machines to boost "Machine King" when "DNA Surgery" is active. "Ojama Trio" is a good choice, since it fills 3 of your opponent's monster card zones with useless monsters that are ideal targets for attacks. "Final Attack Orders" can also be used to inflict more damage, while giving you more ATK, especially if "Ojama Trio" was played, because it forces all monsters on the field into Attack Position, creating more opportunity for heavy battle damage.
Since all monsters on the field will be Machines, you can use "Acid Rain", or, more preferably, "System Down" if you need to clear the field of monsters.
"Chimeratech Fortress Dragon" can be deadly in this Deck. Because you can use your opponent's Machines as Fusion Materials for that card, when "DNA Surgery" is on the field you can clear your opponent's field for a direct attack. "Proto-Cyber Dragon" or "Cyber Dragon Zwei" will allow you to use this effect more often. Either way, 1 "Cyber Dragon" is a great help. Perhaps the most over-looked card for this Deck, ironically, is "Perfect Machine King". With no real easy way to Summon it, and no inherent effects that protect it from your opponents cards, it seems like a waste of Deck space. But with 2700 ATK plus an additional 500 points for each Machine on the field, squeezing it into your Deck might not be a bad idea. If you decide to use "Perfect Machine King", then cards such as "Cyber Phoenix" are necessary, as "Cyber Phoenix" protects all your face-up Machines from Spells and Traps that target, so that cards such as "Brain Control", "Sakuretsu Armor" and "Dimensional Prison" become useless against your Machines.
This Deck is a good counter to Type-specific Decks, such as Plant, and to Swarm Decks, as they will simply fuel the effect of "Machine King". "Uria, Lord of Searing Flames" is also somewhat compatible with this Deck, as there are many Continuous Trap Cards that are crucial to this Deck. "Light of Intervention" will also cripple most Rock Decks.
However, this Deck is weak against mass Trap removal, which is quite common. "Perfect Machine King", in particular, is a Level 8 monster, making it somewhat hard to Summon considering it has no uniquely easy way of Summoning. It is also vulnerable to most card-destruction effects, as it's one of the few newer two-Tributes with no innate effect negation and destruction. "Machine King", as well, is particularly weak even with its effect, as it's ATK is lackluster to begin with and its boost is not that substantial.