"The first account of a mage summoning a full-on person rather than a familiar was first recorded centuries ago. When the individual standing in the summoning circle opened their eyes, their first words were, 'I ask of you. Are you worthy to be my master.' To this day, that servant's identity remains a mystery. The fate of the mage that summoned them is also clouded in mystery." -Fae, Fiends, and Familiars. 3rd Edition  

Reborn Legends

All servants share a commonality of having a known presence in either history or legend. Famous kings and warriors from fiction or history can be potentially summoned as a heroic servant. It matters not on whether the individual actually existed or if they're even from another universe, having a notable presence is enough to have a figure come alive as a servant.  

Magical Pacts

In order for a servant to even exist, they have to be summoned by a mage that's known as a summoner. Summoners and servants usually share some type of common goal and/or the summoner has a particular item that is related to the servant. Once the servant is summoned by the summoner, the pact is seals and the summoner becomes the master of the servant, marked by a set of Command Seals etched into the back of the summoner's hand.   The servant's duty is to accompany the summoner and protect them with their life. The summoner/master's duty is to provide the magical energy that the servant needs in order to exist. If the master somehow dies, the servant will slowly begin to disappear, as their main source of energy is cut off.   The pact that is made is shown via the Command Seals found on the back of the summoner's hand. Command Seals act as a form of insurance to the master, as bounded servants are obliged to serve their master. However, once all Command Seals are used, the contract is closed off and the servant is no longer under any obligations or bindings to serve their masters. There have been a handful of cases where, after a master-servant contract ended, an abused servant promptly turns against their master, killing them before disappearing themselves. Alternatively, if both still wish to remain as companions to each other, the summoner can continue to supply magical energy to the servant.  

Kings and Queens of War

Tynaria Alt's infamous Holy Grail Wars make great usage of heroic servants. When a faction is inducted into the Holy Grail War by the Grail, they are usually given two "representative servants." These two servants are particularly powerful, as they serve under an entire faction, rather than under a single individual. They have better materialization abilities, and their special Noble Phantasms are able to be activated without the need of a single individual providing magical energy, as they more or less take in the collective energy of the faction itself.   At the same time, representative servants are the most vulnerable due to their position of being described as the King and Queen piece of a chess board. If a faction were to lose their two representative servants, the faction pretty much loses the war. As such, representative servants are usually kept away from the public eye for as much as possible in order to do intense training regiments until the time comes to activate the trump card.