The three legendary demons' forms looks like the three Egypatian gods in Yugioh DM (Duel Monsters).
The 3 Legendary Demons are said to to bring world destruction when the 7 keys are taken to open the 7 star gates.
There are 7 duelists that are chosen by the principle who each holds the 7 keys. They must fight the 7 stars, who would like to see the 3 legendary demons brought to life.
The names and deck types of the seven assassins:
The 1st assiassin: Darkness (Tenjouin Fubuki) - Dark Dragon Deck
The 2nd assiassin: Camilla - Vampire/Immortal deck
The 2rd assiassin: Taniya - Amazoness Deck
The 4th assiassin: Don Zaloog - Dark Scorpion Deck
The 4th assiassin: Abidos - Pharaoh's Servant Deck
The 6th assiassin: Titan - Shadow Demon Deck
The 7th assiassin: Emmanuel - Alchemic Deck
The names of these demons aren't official as of yet.
Desceding Thunder Emperor Hamon - Koudanou Hamon
Illusion Demon Emperor Ravel - Genmaou Ravel
God Flame Emperor Urima - Shinenou Urima