From left to right: "Hamon" "Uria" and "Raviel".
The "Sacred Beasts", known in Japan as the "Three Phantom Demons" (三幻魔 Sangenma) are a series of three powerful monsters, with the first three being released in Shadow of Infinity. They are "Uria, Lord of Searing Flames", "Hamon, Lord of Striking Thunder" and "Raviel, Lord of Phantasms". They are used by Kagemaru and Yubel in the Yu-Gi-Oh! GX anime, with the latter also using the fourth monster - "Armityle the Chaos Phantom", their fused form.
Each monster resembles one of the Egyptian Gods and are seen as dark counterparts to them, in a similar vein to the Wicked Gods (though unlike the Wicked Gods, they are never stated in-series to have any relation to the Egyptian Gods). In gameplay, they also have similarities to the Egyptian Gods. For example, in order to Summon one, the player must send three cards of a certain type to the Graveyard depending upon which monster the player plans to Summon. Their Summoning conditions are much easier to be fulfilled.
The designs of the Sacred Beasts, "Uria", "Hamon" and "Raviel" were overseen by Kenichi Hara. Each of them resembles one of the three Egyptian Gods; "Uria" finds a counterpart in "Slifer the Sky Dragon", "Hamon"" in "The Winged Dragon of Ra", and "Raviel" in "Obelisk the Tormentor". Also, like the God Cards, all three Sacred Beast Cards have the same background. Unlike the God Cards, however, they possess certain visual cues that make them distinct from their predecessors, including translucent wings and more pronounced claws and skeletal features.
The three Sacred Beasts are named after characters in Judeo-Christianity. Uria's name is an alternate spelling of the name of Uriah, a man who was murdered by King David because David had an affair with Uriah's wife. Uria may also be a reference to the archangel, Uriel, whose name means "God's Flame", and who guards the flames of Purgatory. Hamon's name originates from the Book of Esther (Hamon is considered an alternate spelling of Haman, a villain who tried to depose the King of Persia and kill all the Jews in Persia. Also the word Hamon means blade pattern, referring to the wave like patterns found on Japan's iconic weapon). Finally, Raviel's name is an alternate spelling of the name of Raziel, the angel of mysteries. Armityle, the fusion of all three Sacred Beasts, is named after Amitiel, an angel of truth in Judeo-Christianity, who some consider to be an archangel.
In Yu-Gi-Oh! GX, the Sacred Beasts are legendary creatures whose cards were sealed by Kagemaru underneath Duel Academy behind the Spirit Gates, which requires seven Spirit Keys to open, as he could not unlock their power at the time. Should they ever be set free from their stronghold, chaos and darkness will envelop the world, destroying everything.
The Shadow Riders sought to acquire the Spirit Keys and revive the Sacred Beasts. In order to open the gate, a strong Dueling presence is required; if the keys are stolen and placed in the gate, (as attempted by Don Zaloog) for example, nothing will occur. To meet the challenge of the Shadow Riders, seven of Duel Academy's finest Duelists were selected, these Duelists were Jaden Yuki, Zane Truesdale, Chazz Princeton, Alexis Rhodes, Bastion Misawa, Lyman Banner (the only Shadow Rider to get his key by default, due to the fact his true identity was unknown at the time) and Vellian Crowler, and although the Shadow Riders were defeated, Kagemaru managed to awaken the Sacred Beasts because of the Dueling energy released from the Duels. In exchange for freeing them from imprisonment, the Sacred Beasts slowly returned Kagemaru to a youthful state, empowering him with the energy sapped from Duel Monster spirits, but Jaden's interference resulted in the man's downfall. They were thus locked away once more.
After Duel Academy is transported to an alternate dimension in the third season, a Yubel-possessed Marcel Bonaparte retrieves the Sacred Beasts and builds a Deck around them, which boosts the Summoning speed of the Sacred Beasts and also supports the Summoning of their combined form, "Armityle the Chaos Phantom". When Jesse Anderson was possessed by Yubel he reappears after having been missing for he uses them in a Deck based around quickly Summoning the Beasts and "Armityle". Although Yubel didn't use them in her final Duel with Jaden, "Raviel" was shown to be in her Deck. After Yubel merged with Jaden, they are safely returned to their resting place.
A Hamon Deck is based around Summoning "Hamon, Lord of Striking Thunder" using Continuous Spell Cards. This type of Deck is best used with "Crystal Beasts", as, when they're destroyed, they're able to go to the Spell & Trap Card Zone, and are treated as Continuous Spell Cards. It can also be used with stalling Continuous Spell Cards like "Messenger of Peace" and "Stumbling".
A Uria Deck focuses on Summoning "Uria, Lord of Searing Flames". This Deck utilizes many Continuous Trap Cards like "Gravity Bind", "Embodiment of Apophis", "Curse of Darkness" and "Solemn Wishes". If you plan on using a hybrid Uria/Fire Deck, "Backfire" can be used to burn your opponent until you Summon Uria. Alternatively, one can simply use "Uria" in a "Lightsworn" Deck, due to the large number of "Lightsworn" Continuous Traps, as well as the fact that "Lightsworns" send cards directly from the Deck, possibly milling many Continuous Traps.
To power up "Uria", cards like "Reasoning", "Monster Gate", "Emergency Provisions" and "Gryphon's Feather Duster" can destroy or discard Continuous Traps. "Anti-Spell Fragrance" can be used to force the opponent to Set Spell Cards, which "Uria" can destroy.
The drawback to this Deck is that it relies heavily on Traps, so "Jinzo" or "Royal Decree" will completely cripple it. However note that "Royal Decree", unlike "Jinzo", does not negate the activation of your Continuous Traps, so they will stay on the field, thus allowing you to still Summon "Uria".
A Raviel Deck focuses on Summoning "Raviel, Lord of Phantasms". Due to the effect of "Raviel" and its Summoning cost, the Deck has many Fiend-Type monsters. It can also afford monsters like "Giant Orc" to give "Raviel" a huge ATK boost once Summoned. Raviel also does very well in a "Dark World" Deck, as the "Dark World" cards can be Special Summoned easily.
A good combo is to get "Raviel", "Dark Ruler Ha Des" and "Doom Shaman" on the field at one time. "Doom Shaman" can Special Summon powerful monsters from your hand to be Tributed to boost the ATK of "Raviel" and "Dark Ruler Ha Des" negates the effects of cards destroyed by Fiends.
This Deck type is a hybrid of the three previous decks and is focused on Summoning the three Sacred Beasts and their fusion, "Armityle the Chaos Phantom". This Deck also uses cards that allow the three Sacred Beasts to function effectively.
Use cards like "Mad Reloader", "Hand Destruction" and "Foolish Burial" to send the Sacred Beasts from your hand or Deck to the Graveyard, allowing you to use the effect of "Phantom of Chaos" to banish the Sacred Beasts and Summon "Armityle". "Elemental HERO Prisma" is also useful in this respect: he is able to discard a Sacred Beast and be treated as that monster.