Fate/stay night is a Japanese visual novel, which was originally released as an adult game. Since its release in 2006 its been adapted into a manga and multiple anime series.
The story takes place in modern Japan. Where Shirō Emiya, became the sole survivor of a massive fire as a child. Shirō was adopted by a retired mage named Kiritsugu Emiya and taught the basic principles of becoming a mage. Kiritsugu Emiya, is the main protagonist in the fourth Holy Grail War (featured in Fate/Zero) which inadvertently destroyed Fuyuki City and left Shiro orphaned. Kiritsugu Emiya died some years later after instilling a strong since of justice in Shiro.
As a teenager Shiro witnesses a battle between servants and almost gets killed. By luck or fate he gets saved by a classmate and unwillingly becomes a participant in a death tournament called the Fifth Holy Grail War. It's bloody warfare where mages use heroes called servants from every time period in history. For a chance to have their wish granted by a cup called the Holy Grail. The stronger the mage the stronger the type of servant they can summon.
There are seven different class of servants. Rider, Assassin, Saber, Archer, Lancer, Caster and Berserker. Each servant is equipped with a special weapon or ability called a Noble Phantasm. Since the Noble Phantasm can reveal the users identity, most servants refuse to use it until moving in for the killing blow or to escape a desperate situation.
Although most Noble Phantasms are weapons like Sabers, sword or Lancers, spear some Noble Phantasms are an ability. Berserker, who is the reincarnation of Hercules is equipped with the Noble Phantasm called regeneration. Since he had to complete twelve labors, he can be reincarnated twelve times before being killed. Gilgamesh, is unique because he is the first hero in history and possesses a very large number of Noble Phantasms. He was a servant of the Archer class who had a strange obsession for Saber.
Saber, is King Arturia Pendragon more famously known throughout history as King Arthur. She is the strongest servant in the knight class which consists of Saber, Lancer, and Archer. Her Noble Phantasm is the mythical sword Excalibur. The sword remains invisible until Saber calls its name to deliver the killing blow to an opponent. The scabbard of Excalibur, is named Avalon. It gives the holder a limited form of immortality by repairing mortal wounds in an instant. Avalons regenerative abilities saved Kiritsugu Emiya's life several times during the fourth Holy Grail war.
Unlimited Blade Works follows a two-week period in Shirō Emiya's life. He goes from an amateur mage to becoming a Master in the fifth Holy Grail war. There are seven Masters for seven servants. Masters are capable of controlling servants by command seals placed on the back of their hands. A Master only has three command seals to control a servant. Once all three commands are used the Master loses control of the servant and can be killed by them.
Shiro, summons Saber and together with the assistance of another mage named Rin Tōsaka tries to stop the Holy Grail war. Tosaka summons the servant Archer who turns out to be the future version of Shiro from a different timeline. Archer hates Shiro because he knows what awaits him after death. His biggest desire was to kill Shiro before he becomes a champion of justice and is reincarnated as a servant.
Servants, maintain their form by feeding off their Master's Mana (spiritual energy). They can heal injuries and use their Noble Phantasm multiple times depending on how much Mana their Master can produce. Some servants like Caster, can drain Mana from humans causing them to fall into a coma and in some cases death.
Caster, is a special servant because some Casters are also mages. This is dangerous to the Master because if the Caster is stronger than the Master...he will be killed. In the fifth Holy Grail War. The Caster is Medea. A sorceress who was the daughter of Greek King Aeëtes of Colchis. Medea was so powerful she was able to summon her own Assassin servant to assist in the Holy grail War. Only the Caster is capable of summoning another servant.
To date there are three different anime series based off the Fate/Stay Night novels. There's Fate/Stay Night, Fate/Zero (the 4th Holy Grail War), and Fate/Stay Night: Unlimited Blade Works (the 5th Holy Grail war). All three of the anime are superb. Although the writing, battles and character development in Unlimited blade works seems to be a step above the other two installments.
If I had to describe the series in one sentence it would be. Fate/Stay night, is a mix of action packed battles, betrayal, tragedy, magic, with excellent characters and plot. Check out the clips below featuring battles between Shiro vs Gilgamesh and Archer vs Lancer.