Eidolon is a fusion-based archetype which focus on using your opponent's graveyard as your fusion materials. The Eidolon cards seem to originate from Sorceries and Alchemy.
Dimensional Barrier can single-handedly shut down any deck that is over-reliant on a single summoning tactic. This prompted plenty of players into diversifying their deck. Hence, more and more OCG players are converting to Eidolon Zoodiacs instead of the mainstream Pure Zoodiacs.
Apart from that, Eidolons, with their graveyard-manipulation ability, counters graveyard-reliant decks such as Infernoids that are rising in popularity.
Aleister the Eidolon Summoner
Aleister is needed as Fusion Material for all Eidolon Beasts (Eidolon Beast are the Fusion Monsters) except for Elysion. Aleister's second ability can be used for OTK pushes.
Eidolon Summoning Magic
Eidolon's unique Fusion Card. Summoning Magic only can perform a fusion using monsters from hand to summon Non-Eidolon Beast Fusion monsters, like a typical fusion summon. However if you were to fuse for a Eidolon Beast, you can banish materials from the field and either player's graveyard. Additionally, you can shuffle this card from the graveyard to add back a banished Aleister to hand.
Summoning Magic allows you to recycle itself and salvaging back your searcher to open up for next turn plays. Since Summoning Magic allows you to banish cards from your opponent's graveyard, try to aim for key cards in their deck.
Reckless Magic Circle
Reckless Magic Circle when activated, tutors for Aleister. When Magic Circle is on the field, your opponent can't negate any Fusion spell card nor can they respond to the summons of Fusion Monsters.
Magic Circle to Aleister is your 'Tenki' to Zoodiac Marmorat and you should max out on Terraforming to increase the deck's overall consistency. Magic Circle prevents your opponent from negating your summon or threatening you with Torrential Tribute. However, do note that they can still chain Dimensional Barrier to your summons.
The Book of the Law
The Book of the Law tributes one of your Eidolon Beast to summon another one with a different Attribute.
The Book of the Law sees no plays currently as fusion summoning is an easy task in the deck. Eidolon decks can also push for damage with relative ease, thanks to Aleister's stat boosting ability. Hence the quick play spell isn't being used much in Eidolon variants.
Magical Name - "To Mega Therion"
This card allows you to Special Summon any number of banished Eidolon Beast.
Mega Therion is bad due to two reasons.
1) It has a freaking long name and is troublesome to type.
2) Since when will you have so many banished "Fusion" Eidolon monsters?
Another time perhaps.
Eidolon Beast Caligula
Requires: Aleister + 1 DARK Monster
When Caligula is on the field, both players only can activate 1 Monster effect per turn. Both players only can attack with 1 monster per turn.
Initially, this card seems rather decent until you notice a major flaw. Caligula has very weak stats. Also, Caligula requires a DARK monster which isn't very common in the current meta.
Eidolon Beast Raideen
Requires: Aleister + 1 WIND Monster
During either players turn, you can target and flip an opponent card face-down.
A decent disrupter as Raideen is a walking "Book of Moon". However with the recent banlist hitting Speedroid Terrortop to 1, the availability of having a WIND monster have significantly decreased. Although now there are lesser WIND targets, one can opt for the Wind Witch Eidolon deck for longer lineup of Wind Monsters.
Two Raideen can also xyz summon into Wind-Up Zenmaioh after flipping two targets into face-down defense position.
Eidolon Beast Cocytus
Requires: Aleister + 1 WATER Monster
Cocytus cannot be destroyed or targeted by opponent's card effects. Cocytus can attack while in defense position, applying it's ATK for Dmg Calculation.
Hmmm decent effects but it's main purpose is to banish Ronintoadin to summon Cocytus. However with the recent limitation of Treatoad, Cocytus seems to see no play. Even before that though, space constraints made the lesser-seen WATER attribute fusion less of a priority to add into the extra deck.
Eidolon Beast Purgatorio
Requires: Aleister + 1 FIRE Monster
Purgatorio gains 200 ATK for each card your opponent controls and it can attack all monsters your opponent controls once. Purgatorio applies Piercing Damage.
Purgatorio is the OTK card as Aleister can boost is to a minimum of 3300 ATK. Purgatorio is used to banish your opponent's Infernoid cards to cripple their resources. Also, it punishes your opponent for leaving a wall of special summoned monsters, thinking that they can do them any good.
Eidolon Beast Magallanica
Requires: Aleister + 1 EARTH Monster
No Effect...just a huge beatstick
Magallanica is necessary for the Eidolon Zoodiac Combo. Since Magallanica is EARTH, you can banish your opponent's Zoodiac Marmorat to cripple their Marmorat engine.
Eidolon Beast Merkabah
Requires: Aleister + 1 LIGHT Monster
When a card is activated, Merkabah can discard a card of the same type (Monster/Spell/Trap) to negate the activation and banish that card once per turn.
Merkabah in my opinion is the best Eidolon Beast to summon out. Merkabah will always have a 'bullet' to negate by discarding the Aleister previously salvaged by Summoning Magic's effect. However, since LIGHT monsters are not readily available, Photon Thrasher and Ghost Ogre are played. Some players turned towards using the LIGHT Kaiju monsters.
Eidolon Beast Elysion
Requires: 1 Eidolon Beast + 1 Extra Deck Monster
While Elysion is on the field, it is all the six Basic Attributes. During either players turn, you can banish one Eidolon Beast from your field or graveyard to banish all monsters your opponent controls of that attribute.
Elysion can banish itself to banish all of your opponent's monsters as Elysion is all six Attributes packed into one. Unless, your opponent is playing some Divine deck.
Eidolons can be tech-ed into many deck, being rather successful. Here are some examples and explanation.
Zoodiac Eidolon like above mentioned, synergize by being able to cope under Dimensional Barrier. Zoodiac Eidolon also have a rather nasty first turn combo too (will be later revealed below) . Since Zoodiac is a rather generic engine to most deck, it works with Eidolon too.
Kaiju Zoodiac Eidolon
Well...so why the Kaijus? Although Kaiju Zoodiac is already a thing and I'm sure you all know why it is played, Kaiju have some sort of a synergy with Eidolon. Since cards like Merkabah have difficulty getting it's LIGHT fodder to summon him, Kaiju provides the fodder after being destroyed. Slumber immediately gives you whatever Attribute you are in need off.
Artifact Zoodiac Eidolon
Artifacts are an OP Floodgate against Meta that's all......and Artifacts are LIGHT for Merkabah plays.
No no no. This is not what all of you are thinking. WWE stands for Wind Witch Eidolon. Although I am not 100% sure why it is played, the Wind Witch build gives the deck options to go for 1 card Crystal Wing Synchro Dragon. This year's ACQ Winter 2017 is won by an Artifact WWE deck.
That's all for today folks. Stay tuned to Ken Sir's post from Beyond the Duel regarding the combo plays of Eidolon.