Thank you for your detailed response! I was envisaging a Legion Battlemage, so a frontliner was my intention, and I was planning on Mage 10/Runeknight 10.
One thing: in the approach outlined above you've dropped Favoured Weapon, a prerequisite for Runeknight.
Ok... somehow I totally dropped the ball on Favoured Weapon.
So, here is the corrected version:
Fighter: Armor Basics
1: Armor Mastery
3: Favoured Weapon
Mage 4: Hidden Spells
6: Armor Supremacy
9: Sword Basics
12: Darting Weapon/Charging Basics (whatever you prefer)
15: Sword Mastery
18: Sword Supremacy
Mage 8: Casting Time - Casting Time is quite nice to have as a ace in the hole with the doubled casting times from Difficult Magic.
Basically, you have to make a choice between Darting Weapon or Charging Basics with that build. Both are good, but if you want to keep the line, Darting Weapon might be more appealing. It also has no limitations on how often it can be used in combat. On the other hand, if things like high-risk & high reward do the job for you, you might consider Charging Basics.
Regarding Spell Secret: I was intending to use it to grab a couple of out of reach spells. M/R gets to Circle 6, so I was going to pull a couple of 7s down to 6. Yet everyone seems to focus only on Spell Secret's uses for Arcane Wellspring. Having not yet played a game, I was wondering if Arcane Wellspring was really that good, especially since I'd be getting it at level 20 with this build.
Let me preface this with: It's just my opinion... naturally your's might differ.
You get exactly two Spell Secrets to pick over your entire career... and quite late at that it seems.
Additionally, the Mage gives you spellcasting abilities up to level 4, while the Rune Knight provides 2 more on his third and seventh level.
Furthermore, you will end up with a total of 30 Spellpoints.
With those things taken into consideration: Why do you want two level 7 spells brought down to level 6? And which would that be?
I mean - depending upon your spell selection - you are either fire support (quite likely), someone to forge favourable circumstances or a healer... quite likely a mix of several.
You have plenty of options to kill people with your mind
and the same goes for any option really.
Level 6 spells in Cloak&Dagger must be cast against a DC of 36... for you those are not reliable spells, thanks to the setting.
You are excellent at kicking ass, slinging spells and taking names, but I'd concentrate on spells of level 5 and lower primarly... just don't forget to pick some nuclear options from spell level 6 like Lightning Bolt II, Cause Wounds II (freakin' mook killer), Mass and Quake Touch if you want to be living siege equipment.
A lot of your mainstay combat spells could be low level spells that are reliable and efficient from the spell point side of things.
Attack Spells recommended for that are: Ray of Enfeeblement, Scorching Ray, Searing Ray, Polar Ray I & II
Area effect spells: Fireball I, Call Lightning I and II, Lightning Bolt I. Spells like these will often be saved against, due to your lack of spellcasting feats, but even so they will do damage and take care of mooks.
Use ray spells to deny them saves and hit single targets for maximum effect.
I just browsed through the core book... you might have other sources for spells available.
Other spells: You should have plenty of spells over to pick up anything else that you like or your group might demand.
On the topic of Arcane Wellspring... yes, it is awesome. The ability to cast level 1 spells over and over again is BIG.
Combined with two spells brought down to level 1 by spell secrets and you are set to inspire fear into your enemies hearts.
Look at it this way: A Fireball II can inflict up to 16D6 damage... your Mage/Rune Knight could use that spell 5 times at level 20 and then he'd be out of gas.
Sure he could do inflict a maximum amount of 480 points of damage, but that's the most generous assumption.
Enemies more often than not are capable of dealing with saves, therefore a LOT of damage would be negated, which is of no concern against mooks anyway... usually.
Now let's say your Mage/Rune Knight picked up Scorching Ray - a second level spell - as one of his spell secret. Not only is this spell easier to cast, it's cost's are also cut in half... so even without Arcane Wellspring in action you can cast that spell more often. The only disadvantage this spell has is that it can only be cast in short range... which luckily plays into your strengths and because it is attack-based there is no save to worry about AND you can hit up to 3 different targets or just focus fire. Sure, the spell deals only up to 6d4 per ray damage, but you can cast that spell more often. There is no save, it costs 1 spell point to cast if picked as a spell secret and with Arcane Wellspring you can use it all day long after you are out of spell points.
In the end my reasoning might not be your cup of tea and therefore I'll say this:
Overall, go with what you like. There is no right or wrong. If you see a spell that you just want to have... pick it.
The world won't be doomed for it... I think.
The bottom line is: Have fun.
Sometimes you have to slay the princess and save the dragon.