Helheim exists as an Early Age nation alongside Vanheim in the Age of Vanir, but later joins with them as Midgard in the Late Age. Helheim is one of several Dominions nations that are based off of Norse myth.
Some people feel almost guilty playing Helheim in single-player because it's so ridiculously powerful against the computer. The combination of (moderately expensive) stealthy, high-defense units with powerful Earth/Death magic can make the game very easy. Against human players, there exists some controversy over whether Helheim is overpowered or merely easy to use effectively. Be warned, but if you're just learning Dominions 3 Helheim can be a really fun pick.
A traditional Helheim pick is dual-blessed W9F9, for the defense boost (which synergizes with Glamour to make Valks/Helhirdlings/leaders almost impossible to hit), quickness, and attack bonus. Since Valkyries especially tend to tire swiftly in long battles, if you're intending to do a lot of air raiding you might take W9E9 instead for the recuperation benefits and extra Protection--against Early Age garrison units F9 is overkill. Valks/Helhirdlings are capital-only, but so cost-efficient that the majority of your income for the early turns can go into researchers/gem searchers while the Valks/Hirdlings take care of expansion, so by the time you run into heavily-armored opponents you'll have death magic (Drain Life, Shadow Bolt, etc.) to take care of them.
All Helheim units but the Serf Warrior and the Svartalf come with the "glamour" ability, which makes them fairly safe from close combat attacks, but doesn't provide protection against ranged attacks and spells (Since patch 3.08).
Note that the Helhirding is listed twice: once as a capital-only unit and once as a nation-specific unit that can be recruited anywhere. There is no difference between the two. This is likely an error in the game code, but what it means is that you can recruit the sacred Helhirding from any castle with a temple, leading to the potential for a massive block of sacred cavalry by mid-game. [This is fixed in the 3.01 patch]
Helheim is very strong in death magic (commanders up to Death-4, and nearly all commanders have at least Death-1) and Earth magic (up to Earth-3). Since you'll be buying lots of Svartalfs (up to Earth-3 or Death-3) to do research anyway, you may as well take advantage of spells like Blade Wind and Curse of Stones. This makes Evocation-4 a priority, as well as Conjuration-3 (for Summon Earthpower) and Enchantment-3 (for Raise Dead/Skeleton). Alteration (Soul Vortex, Drain Life, eventually Disintegrate), Enchantment, and Conjuration are strong schools for Helheim.
Helheim comes with two starting sites, Helhalla and Gnipahala. Helhalla provides one air gem per turn and allows recruitment of the capital-only Helheim units. Gnipahala provides four death gems and one earth gem per turn.
Surprisingly, Vanlade is one of the few Helheim commanders to have the "Sailing" ability -- and given that Helheim has no way to get Water, he is one of the few options short of items to get across a water barrier.
On your first turn, turn the tax rate up to 200 to double your income while your starting army patrols to reduce the unrest. On the second turn, send out that army to capture some territory. Huskarls are pretty tough, especially if you added a few Hirdman on your first turn, so that army should be able to take several territories before it starts deteriorating. Meanwhile be scouting out soft lands with your starting scout, and building one to three three armies of 8-10 Valkyries each led by a Dis to take those lands. If you're playing the dual-bless strategy, be aware that the area of effect of bless is pretty small (5 squares, about 2x2) and it's easy to spend 2 turns blessing and only hit 7 of 10 Valks. The Dis seems to have an easier time blessing two squads of 5, slightly separated from each other. For this reason, if you don't need the flying Dis to lead a Valk army, consider using a Vanjarl (priest-2) to lead armies of Helhirdlings. Since Valks and Lings are pretty cheap, you'll be able to purchase same Svartalfs to start research/gem search on several early turns, and save up money to buy temples and a new fortress. You should have conquered 15-20 provinces by the spring of year 2. They won't have solid defenses yet, but your HHs and Valks are mobile, so that won't be a big deal.
Helheim has some of the best cavalry in the Early Era. Masses of light horse with the glamour ability means that you can whip across the field without any real fear or arrows or artillery. The fact that one of these cavarly units is both sacred and (pre-3.01 patch) available for purchase at any of your castles (with a temple) means that you could theoretically pull off some really wicked bless strategies that will serve you well into the late game and only adds to the potential for mayhem.
There are several things that differentiate Helheim from Early-Era Vanheim:
- Vanheim's Dwarven Smith is more powerful in Earth magic but less so in Death and without the bonus 10% random. Perhaps more importantly, Helheim's Svartalf's are recruitable anywhere and Dwarven Smiths are capital-only.
- Helheim has a Vanherse-like Helkarl with D1 magic instead of A1 and 1 point more of Strength, and a sacred Hangadrott which is like a souped-up Helkarl with up to D4 or A3.
- The somewhat puzzling lack of the "Sailing" ability
- Heavy cav/flying sacreds vs. Vanheim's berserkers (Helheim's high-defense sacreds synergize better with glamour, especially since berserk only kicks in after taking damage)
- Vanheim has (capital-only) immortal pigs with trample
In this writer's humble opinion, Helheim is clearly a stronger nation than Vanheim on large maps unless you're gunning for E5 spells like Petrify or have a good use for berserkers. Helheim is very strong with either a high bless strategy (F9W4D4 for example) or to take advantage of excellent Vanir heavy infantry with buffs like Legions of Steel and Strength of Giants. In large games as soon as a fort is built it can start cranking out Hirdmen and Svartalfs--even 15 Hirdmen and 2 Svartalfs is a potent force when buffed--whereas you'd need to rely on Boots of Flying or spells like Cloud Trapeze to ferry Dwarven Smiths to the front.
Did I mention that I love Svartalfs? Once you have Conj-3, Ench-3, and Const-3, any Svartalf is capable of casting the basic buff sequence <Summon Earthpower, Legions of Steel, Strength of Giants> that turns Hirdmen into Abysian-class heavy infantry with glamour and defense 17+ to boot. Svartalfs come in four classes because of the extra random. Each has its uses after the basic buff is done.
F1E2D2: Good battlemages. Summon Earthpower, Magma Bolts/Eruption. Requires Evoc-3/6 respectively. 26 points of damage to AOE 5 works well against any infantry, even fire-immunes (it's not fire damage). Doesn't always hit moving targets but adds extra punch to your awesome frontline infantry. If you give one a Skull Staff he can spam Bane Fire at Evoc 6, which rocks, frankly.
E3D2: Summon Earthpower, Earth Meld, Blade Wind, Armor of Achilles/Destruction. Requires Alt-2, Evoc-4, Alt-2/4 respectively. Nicely counters either high defense or high Protection. Also suffers less fatigue casting the basic buffs because of the extra earth level. With Earth Boots they can cast Petrify at Alt-6.
E2D3: Skelly spam. Since you've got Enchantment-3 anyway, you might as well script all your D3 Svartalfs to Raise Skeletons x5. Skeletons are no better than decent, but they're free troops, and they can be buffed by the LoS/SoG combo just like the rest of your troops. If you prefer artillery, Shadow Bolt and Shadow Blast are also options, and at Thau-5 they can spam Leeching Darkness. With a skull staff they can cast things like Darkness (Alt-6), Drain Life (Alt-5), and Rigor Mortis (Ench-6).
A1E2D2: For a long time I didn't know what to do with these guys beyond using them as inferior earth or death mages. Then someone pointed out to me that the +350% affliction chance from death-9 applies to spells as well as weapons, and suddenly... The E3A1 Evoc-7 spell Rain of Stones claims to be pretty much equivalent to Blade Wind except that it covers the whole battlefield. It's not. Unlike Blade Wind, shield-equipped units still take heavy damage. It's usually not enough to kill them, but most units in the army will take 2-7 points of damage or so even with Prot 12. Combine that with a D9-blessed Svartalf (which requires astral mages to forge Shrouds of the Battle Saint for 5 pearls, available at Const-4) and pretty much the WHOLE BATTLEFIELD is going to be afflicted [see footnote 1]. Of course that includes your own forces. I can therefore see two scenarios:
1.) Soften up enemy armies the turn before attack with a blessed air Svartalf scripted to <Summon Earthpower, Rain of Stones, retreat>. If you want to save the Svartalf, add Ironskin in there first and give him two Earth gems instead of one to reduce his fatigue enough to still be able to retreat. Or write him off and save the gems. Cost: 5S, 1-2E, in exchange for annihilating an enemy army of arbitrary size as a useful force. [This overstates the case. Most afflicted units are still quite useful.]
2.) Research Enchantment-4 and use a Winged Helmet or Bag of Winds to boost him to A2. Then cast Cloud Trapeze to drop said Svartalf onto an enemy army in the magic phase before normal movement takes place. Follow up if possible with a normal army attack on the same turn to wipe out the pathetic remnants--they'll still be low on hitpoints and probably morale. This tactic would work even without the D9 blessing, but the synergy is that even if the enemy out-foxes you in the movement phase (causing your regular army to miss), your Svartalf will have done more than just killing off 20% of the force with Rain of Stones. He'll have crippled the whole army. Cost: 28A, 5S, 1-2E. Hopefully you get to reuse the helm and shroud so the marginal cost is 3A, 2E.
 In tests vs. Marneri Ambibate Nobles, who are pretty decently armored for EA, this tactic resulted in about 25% of the Ambibates dying from the Rain of Stones and another 45% getting afflicted, leaving 30% slightly wounded but largely intact.
When all is said and done, I'm not certain I wouldn't still rather have a D9-blessed fire Svartalf instead, but the Cloud Trapeze potential is nice, and Rain of Stones is a surprisingly nice spell.
Vanir can have real problems going up against E9N9 Niefel Jarls in cold climates. Basically you get two rounds of attacks before they freeze you to immobility, and their armor is heavy enough that it's hard to do enough damage to kill them in those two rounds. It would be nice to think that Destruction would deal with the Jarls--regen without heavy armor isn't very scary--but even though earth mages are ubiquitous for Helheim, it's almost impossible to get them to actually hit the giants with the spell since it only covers a 2x3 area. (And you have to make them hit in the first five, scripted, rounds because the AI never chooses Destruction on its own.)
I haven't been able to find anything, even Construction-7 Weapons of Sharpness, that lets Vanir stand up to Niefel giants in melee. The best tactic I've found so far is Alteration-6 (allows Darkness and Drain Life) combined with Enchantment-3 (Raise Dead) and possibly Construction-4 (Skull Staff to use Svartalfs instead instead of Hangadrotts). Have the low-level death mages spam skeletons while the high-level ones suck the life out. You don't actually need any Vanir troops present to make this work; if you have them with you set them all to Guard Commander so they stay out of the way and don't get killed. It's obviously better to maneuver around Niefelheim's forces and cut off his gold supply, but this way you can back up the maneuver with a real threat of killing his pricey units. It's certainly hard enough to do...
Speaking of maneuver, Vanir excel at raiding behind enemy lines of course. Since all of Niefelheim's good units are capital-only, you can use the Alteration-4 spell Blight (E2D1) to raise unrest by 15 per casting and kill off population. However, this isn't nearly as cost-effective as Rain of Toads, which Vanir don't have. Something that works *almost* as well is to sneak into his capital and capture the province (pretty easy vs. militia with dual-blessed Valks or Helhirdlings), and instead of sieging the province, turn the tax rate up to 200 (like always) AND GO BLOOD HUNTING. Since your commander likely has no skill in blood-hunting, this nets no slaves but raises the unrest up to 83 or so in a single turn. If it gets above 100 he won't be able to produce any units at the capital, even after you leave, until it comes back down.
Update: Iron Bane (Alteration-6, E3) does the same job as Destruction, but it's a whole-battlefield spell costing only 1 gem, so range and precision aren't at issue. In a test, 14 W9D9-blessed Helhirdlings were able to take on 4 Niefel Jarls and 3 Niefel Sacreds, plus a cyclops, in cold weather, and kill 1 Jarl while losing 11 HLings. I didn't try one at more even odds, but the one Jarl that got killed was only facing 6 or so HLings and I think this spell did a pretty good job of countering the Jarl's invulnerability. Prot dropped to 9. Note that this affects *all* units on the battlefield, but Glamour helps offset that a little. Still expect heavy losses when fighting Jarls, but it's now doable with conventional forces, if you can reach Alt-6. Note also that this would probably synergize well with Raise Dead and Darkness since undead are still immune to the cold; now they can not only hit Defense-24 Jarls but they can do damage too. Not tested.
On the other hand, Destruction breaks armor (semi-) *permanently*, while rusty armor goes away at the end of a battle. You could try a little one- or two-Svartalf assault (or defense behind 5-7 PD) that goes "Summon Earthpower, Destruction, Destruction, Destruction, Destruction, Retreat." (Summon Earthpower reduces the fatigue by boosting you up to E4. If it's only an E2 Svartalf you'll only get 2 Destructions before having to retreat.
If you were playing against high-defense units like other Vanir, and having trouble hitting, you could try Earth Meld (Alt-2, E2, fatigue 80) which, when it hits, cuts their defense down to 5 or so until they break free.