Recommend a spell-caster custom leader / faction for me

Flames. Flames. FLAAAAAMMMEEESS!!! BUURN THEM ALLL!! TEH HERETICS! THE BLASPHEMERS! BUUUUURN THEM IN RIGHTIOUS (or unrightious) FIIRE!!!

Pariden based sov, the bonuses are...extreme. Make a Warlock +50% damage, weak, take Life, or not and neglect the rest, you can buy the elemental books pretty early in the magic tech tree. You can probably guess the best magic traits but spell mastery per level, I take Hardy, Brilliant, Impulsive, Scholar and Life. Lots of heroes has life magic, but there is no guarantee you will find one any time soon, take the crown, give it to a melee champ and buff the hell out of them, you only need one or two buffs.

Super caster.

Doesn't that only work in your own land?   

Hmm.  What spells do I use to actually fight stuff, though?  I already buff my buffable units as soon as I'm able.  I know there's "hit everything" spells like Blizzard and 1-target spells like Flame Dart, but so many options seem either weak or crippled by long casting times, compared with just shooting enemies with arrows and/or chopping them up in melee.  

Well, I use my sov pretty much solo and use my other heroes to level my troops. Solo melee enemies have to come to you or if mounted with the tireless march trait you go to their ranged attackers. A level 12 fire based sov with the proper buffs (like Mantle of Oceans) is simply a god of destruction. I suppose you might have the same effect by stacking a big archer stack but with high initiative and one of those awesome +100 crit damage daggers you roast...everything. with the act first trait the hilarious thing is, if you are fighting say a dark wizard pack (despair= unhappy archer army if you don't act first), you just cast the 100% resistance to spell tactical buff (only lasts for that combat) and use spell leech to farm mana lol, I love that spell.

Anyway, yes, Fireball is the main damage spell for groups but a high damage fire arrow seriously takes out, well, almost anything.

On my last mage, and I don't know if I was just really lucky or not, I took path of the mage at level 4 and was offered a second path at level 14, it was warrior so I didn't take it, but if you got assassin there...holy crap.

By running around with a solo sov you can really plow through the bad guys, you are not stuck at 3 moves, you are stuck at 5, with the spamming of roads that occurs with all the trade treaties, you can move around quickly or if you properly leveled Air you just cloudwalk everywhere. The big thing with Pariden is that you can be running 4 essences in your lucky cities and 3 in most of the rest, that gets you 6 to 8 mana per city per turn with meditation or whatever that city buff is called. You will have lots of mana to throw around but you do need Mantle of Oceans to really play.

Make a Quendar (Magnar-race) Warlock with Fire, Water, and Death magic, give the faction Flesh-Bound Tome and Death Worship, with a third point to use as you desire (Binding is fun), and spam as many stick-armed slaves as you can cram into your armies.  They don't need to do anything more than die; you will be using them as meat shields, and sacrificing them for mana when they have outlived their usefulness.  Congratulations, you now have the single most powerful spellcaster in the game!  For extra doombringing, give your sovereign Brilliant in exchange for Clumsy, as this character should never be using a melee weapon.  That's what Burning Hands are for!

I'm currently trying to play a game as each of the default factions and sovereigns. After a good game as Altar and a less successful one as Resoln, I chose Magnar, who I've never had much luck with in the past but is proving to be something of a badass this time around.

In the early game I followed much the same strategy as Emperorjarin above, spamming one unit of slaves per turn and using them as meat shields and strategic mana reserves. The other two Flesh-Bound spells never amounted to much, but Cull the Weak was essential until I got my hands on enough shards to sustain my mana pool, and got the gear and abilities to bring the cost of my bolt spells down to spammable levels.

I got quite lucky on my level-up rolls and soon got Evoker 1-3 and Affinity. Once I got Fire 4 I summoned a Fire Elemental, an Imp and a second Fire Elemental using the Imp (best hitpoint I ever spent). Magnar wears the caster's robe that gives a discount to mana costs, rides a warg and carries a sacrificial knife for bonus initiative and crit. He and the two elementals are currently scouring the map destroying everything they see with Flame Dart, Shadow Bolt and Embers; the Elementals tank anyone who makes it to melee range, and Fireball takes care of mobs of city defenders and the like. I didn't have much luck with water casters (I'm levelling a couple now) so no Mantle of Oceans for much of the game.

The only thing that this stack has had trouble with is Obsidian Golems due to their magic immunity; I'm also not sure how I'll deal with the Ashwake Dragon that's lurking around the Eastern end of my Empire as it's immune to fire, but apart from that nothing can withstand my stack of doom. My major rival is Yithril and I'm looking forward to seeing if I can burn all their juggernauts before they get into combat, as I doubt my elementals could withstand much of a beating from them.

The rest of my armies use more normal tactics in battle, Magnar's approach definitely doesn't drag out the battles - pretty much everything gets one-shotted by my various bolts and/or fireball.

tl;dr?

What he said.

Juggernauts are over-rated when you have mainly ranged/magic casting units.

The only real question is - Is there a magic combination that isn't powerful?

Earth+Air = don't get hit, or take little damage if hit.  Plus be able to make the enemies not be able to take their turns.

Fire, Death - kill everything before it reaches you

Water, Life - keep units alive longer?

If you're using a Pariden-based ruler, you need strong cities.  There's no reason to do Pariden if you don't go for the spellbooks on the magic tech progression, which means you'll be doing that much more than other rulers--and dividing your tech advancements.  This means slower tech advancement overall; other rulers will probably reach the world achievements and military upgrades sooner than you.

Such a thing is unforgivable.  You must change that.

The way to do that is cities.  Build research buildings as quickly as you can.  Pump up research so that you can afford to take time out of the other two tech progressions, without falling behind.

But, you'll need money to rush those research buildings, which means you'll also need buildings that bring in cash.  That cash will also come in handy to buy the services of champions, to get mounts, and to buy all those nice spellbooks you'll need to push city production, raise your units' experience in battle, summon elementals, and do direct damage.  And as a side benefit, you can rush mana creating buildings, since you'll need to drop arcane monoliths all over the place to grab resources, especially shards, which increase both the amount of spells you can cast and the power of them.

Personally, I think Warlock is a good trait, though Summoner is good, too, especially in the beginning of the game when you're stuck with a a weak ruler and a (potentially) weak first champion.  Having that warg is pretty nice.  I always get a longbow (I've no idea why they even include the shortbow option at this point, since it has only disadvantages), and Clumsy.  My mage leader doesn't need to be in melee.  Life is a very useful spellbook to have at game start, given that at second level you get Heal.  You may want to consider Earth, too, since Enchanted Hammers means a nice boost to city production.  And eventually, the Earth spellbook yields Growth and Shrink, two of my favorites to use on frontline champions.

Don't neglect grabbing Procipinee's crown, either.  If you're playing a mage sovereign, it's better to put all those enhancements on a champion, and the crown means no mana charge per turn for these.

Other good traits for this build?  Brilliant, and Scholar.  You can never have enough mana, or enough research points.

Well, it is really not that bad. You lost 3 turns to make an early militia with all upgradeable gear and send them off with your sov. I never thought of choosing clumsy instead of weak, but over all it's a better trait, just don't stand close to your tanky unit in the first few fights. Best early game weapon is the practice sword if you can find it

I don't play empire, I never have and probably never will, in any game, I just can't play a bad guy.

I like Air and Water.  Blizzard has a seriously wicked area of effect, and Thunderstorm is a strong single-target spell.  And at the lowest levels you have Haste and Slow.  Being able to act more often is useful, being able to stop the enemy from acting so often is useful too.  If you need your sov to heal you can always take the Blessing of Restoration buff (from the magic tech tree).