The System

A rough outline of the system itself:

Given in the original visual novel, all Servants' abilities are recorded on a "Status" page, with all of their attributes, skills, and ultimate techniques ("Noble Phantasms") listed, briefly described, and given ranks.

As shown in bars next to the ranks of attributes, each rank can be equated to a number as follows:
E=1, D=2, C=3, B=4, A=5, A+=6, A++=7, EX=?
For the purposes of the RPG, I will assume that EX=9

With these ranks given, the main core of the rules can be formed. Having played and run Shadowrun some years in thepast, and currently running a WEG Star Wars campaign, dice pools seemed like the way to go- the numbers aren't flexible enough for straight-up dice modifiers, and further modifiers from various environmental effects seemed too finicky to mesh with the supercharged heroic feel of the game.

So, with that in mind, each Rank is equal to a number of D6s. When a roll is required, two opponents roll against each other with the higher result winning. An equal result shows a stalemate. If there is no literal embodied enemy, the referee will assign the task its own Rank according to the difficulty.

As I had built the system before now, the system became more and more complex, requiring short but unnecessary equations to calculate things like Health totals, Armour Points, Mana pools, and Luck points. In order to avoid this, I decided to strip back the system substantially- allow it to play more on the level of a wargame, and then build it back up to an RPG from there.

As such, each attribute gained only a single function each.

Strength: Damage
Endurance: Damage resistance
Agility: actions per round
Magical Energy: mana points spent on skills/spells/noble phantasms
Luck: bonus points to spend
Noble Phantasm: Attacking/Parrying

Skills became either Passive (applies, whenever indicated, automatically) or Active (requires mana points to activate), and had their effects reduced to simple dice rolls or as augmentations to attribute rolls- boosting attack power for example.

An inspection of the skills list given in the visual novel will show that there are a number of skills which cannot be applied in a wargame-type scenario (i.e. in combat), and as such they do not provide direct mechanical bonuses in the same way, but are open to interpretation by the players and referee, and may or may not require any dice checks. In any case, these skills are almost always Passive.

In my first trial run of the combat system, I grabbed a Paizo blank squared map and some chess pieces, and set to work.

Initiative = Agility roll
Health = can take wounds equal to their Endurance rank
Damage = All successful damage rolls deal 1 wound
Movement per action = 3 squares

Two (white) Knights faced off against six (black) Pawns: Neither had any special abilities, and very standard sets of attributes. Pawns had all attributes of rank D(2), and Knights had all attributes of rank C (3).

After a few rounds, and a wide variation of rolls, the two knights stood wounded but victorious over a field of dead pawns. Some of the pawns lasted a little longer than expected while others died instantaneously under the flurry of blows visited by the knights- whose additional attacks had surely won the day.

The system had the constant charm of all RPGs/wargames with its unexpectedly wild dice throws- some times they would come up all 1s, and sometimes all 6s, and the Luck points which can be spent to boost rolls allowed for some slight variation, but some rounds resulted in nothing happening. Still, with standard attribute sets and no skills it was bland- this was only a test of the basics, mind you.

The second test was the face-off of two figures more akin to the champions that Heroic Spirits (Servants) are. With varied stats and two skills each (with a balanced Ranks total, however) the black King stood against a white Queen.

This match proved more interesting than the bland 1-number stats of Knights vs. Pawns, and I could see the beginnings of strategic differences when it comes to the use of skills and luck points- whether to save them for defence, use them evenly for a slight constant advantage, or pour them into big-shot attacks in an attempt to overwhelm the enemy.

The King won, his higher speed, attack ability and skill use beating the Queen who held more robust strength and endurance. It was a close run thing as well, the King just bearly surviving, and holding off a massive all-or-nothing last minute attack by the Queen with some very lucky rolls.

So, system core established, its time to start fleshing out the canon skills into useable abilities, and to consider the role of Servants' counterparts- the fragile but important Masters, the Magi who control Servants to win the Holy Grail War.