This text is suitable both for completing the campaign as Necromancers and for duels. My experience in duels is still modest, so I hope that you, dear reader, will supplement my findings and show what can be done better.
The Requiem is a necromancer mage characterized by strengthening all Tear reputation skills and two unique abilities:
No rest for the wicked (oh my, a translation of the good old "No rest for the wicked") - strengthens Necromancy by 5% at the first level of Tears and another 2% up to a total of 7% at the second.
That is, using the faction skill, the Requiem raises slightly more of his own fighters from the dead than other necromancers.
At first glance, it seems insignificant. But when choosing the strategy I will describe below, it's simply wonderful. Thanks to this, you, dear reader, can either avoid many losses or — more importantly — turn the tide of more than one duel.
Sacrifice - a selected stack of our fighters takes ALL DAMAGE for all our stacks for a couple of turns until it dies, and incoming damage to this "lucky" unit is reduced by 33%.
This is our "gimmick," the primary target for an enemy dispel every single day. Because while the spell is active, the opponent can only hit one of our stacks — usually the most protected one. And in combination with our other tricks, this essentially means two turns of own unrestrained chaos. I tell you, it's a real bomb.
Why? You'll see now.
First and foremost, skills.
The best way to show the Requiem' same coolness is through an example of a Necromancer duel premade. Thanks to conflux, we create a new dynasty hero.
Physical skills:
Defender 1-2: adds +5 physical defense to the hero. We need sturdy undead capable of playing from defense.
Ambush: for the first three turns, our troops receive +5 initiative. It is extremely important for us to cast a spell before enemy troops move. For duels, our higher vampires (base initiative 50) should go first; for campaigns, insatiable ghouls (base initiative 35) are quite good.
Tactics 1: we can arrange the troops in a convenient order before the battle. At the start, it is necessary to thoroughly cover our most fearsome weapon with defenders. In the campaign, tactics will allow you to clear an insane number of troops at level 1 using a couple of tactical tricks.
Endurance 1-2: adds +6% HP to our troops. This is because we need hardy and plump undead!
Magical skills:
Magic Aptitude 1: +30 mana for the hero. Every unit will count, so this is a must.
Arcane Keeper 1-3: +7 spell power for the hero. When possible to cast one spell per battle round, every spell should be maximally strengthened!
Time Stop: a target enemy stack does not respond to attacks and stalls for up to 4 turns. Cyclops last slightly less. One of the most powerful spells in Heroes VI — and our "finisher," more on that later.
Meditation: restores our mana. And we will need a lot of mana in every duel. A lot.
Mana Drain: steals mana from the enemy hero and transfers it to us. Meditation for the poor — exclusively for prolonged battles, especially against Inferno. M:TG players call this a sideboard. Yeah, that's exactly what it is.
Alternatively, Dispel Magic or same-type own Dark Magic dispel could be here. I don't yet understand which of these fits better!
Weakness, Weakness (global): "global" for 5 turns reduces incoming damage from opponent troops by 20-25%. A harsh necessity against all sorts of warrior heroes in duels. What's nice is that it works on all their attacks — and all types of attacks.
Despair, Despair (global): for 4 turns, enemy troops receive approximately -20 morale at the maximum level. Read as: "goodbye, morale" for mage heroes' troops, "hit me less often under morale, you damn angel!" — for warriors. Our trump card against Haven heroes — less morale = fewer guardian angels!
Summon Dark Elementals: when we need to block enemy shooters, dispel and reduce damage of the main enemy stack (hello, angels, hello, seraphs!).
Dark Magic 1-3: +8 to the power of dark spells. So that the above is at maximum strength.
Summon Water Elementals: I wrote about them in detail in the cleric guide. Briefly: we need them when it's necessary to block enemy shooters and hold particularly dangerous stacks like seraphs, furies, or abominations in place.
Restoration, Restoration (global): the only healing spell available to us. Simply a necessary remedy against enemy area-of-effect strike spells and similar troop abilities like dreamcatchers for Barbarians.
Stone Skin, Stone Skin (global): the best defensive spell in the game. Significantly reduces physical damage to our troops.
Summon Earth Elementals: these guys can turn even the strongest enemy stacks to stone for a turn. We summon them when we need to block shooters and delay key enemy stacks like Radiance or Kensei.
Earth Magic 1-3: for reasons already clear.
Horror, Petrification, or Subjugation would also suit us. However, even without them, a powerful mix of defense and battlefield control has been created.
Now a few words about the troops.
Key stacks for the Requiem are ghosts, liches, and fate weavers. They have a magical attack type; under the leadership of a mage hero, they receive a significant damage bonus multiplied by all our spell power boosters. Yes, in spider form, fate weavers have a magical attack, and in infantry form — physical!
At the same time, liches and fate weavers are capable of dealing huge damage from a distance. They will be our main striking force in duels.
In the campaign, at different times, we will deal damage with bone archers, summoned elementals, and the hero's auto-attack.
I will note separately — our other troop types are nothing more than a cover for liches, ghosts, and fate weavers. An undead shield of thick, sturdy, and well-fed troops, strengthened and reinforced by defensive spells.
Now, actually about the battlefield strategy.
The strategy begins with preparation — and here is what the start of a duel for the Requiem might look like:

In case the image does not display, from top to bottom:
Archliches in the upper corner, vampires before them;
Ghosts #1, insatiable ghouls before them;
Fate weavers;
Ghosts #2;
Rotting Lamassus.
Casting spells depends heavily on the opponent. Usually, against warrior heroes, we will need Stone Skin (global), Weakness (global) and, optionally, Despair (global).
For field control — summoning water or earth elementals. For dispel and damage reduction — dark elementals.
And here is the most pleasant part about Sacrifice.
It should be kept ready for our darkest hour. When our stacks are about to start falling one by one. The main target for Sacrifice — Fate weavers, Lamassus or, closer to the end of the battle — summoned earth elementals. Yes, summoned creatures can be "scapegoats"! For this, they should be summoned at a point on the battlefield far from the opponent and, preferably, covered by other stacks.
Fate weavers are perhaps the best candidate for Sacrifice. They have extreme defense values, many hits — and they can be raised from the dead using Ghosts and necromancy.
This is where it becomes clear why we need No rest for the wicked.
Fortified by our defensive spells, Restoration, and the hero's physical skills, Fate weavers are a tough nut to crack. Which can be instantly restored to its original state using strengthened necromancy.
And Sacrifice, meanwhile, also reduces incoming damage by a third!
In general, there is a lot of synergy here and it's all in our favor.
And about our "finisher." A convenient scenario for us is to gradually kill the opponent's troops stack by stack. Meanwhile, the last one, possibly the strongest — we can put into Time Stop. Over 3-4 turns, there is a good opportunity both to restore troops using healing with necromancy and to prevent the enemy hero from escaping. In the campaign, this technique works splendidly against neutrals consisting of 1 stack, for example, cyclops or seraphs. In a duel, this allows you to finish the game with equal troops by stopping time for the enemy's key stack. The main thing is to lure out the dispel from the enemy — or kill their creatures with dispel first.
Now about artifacts.
Of the dynasty weapons, Soul Stealer suits us perfectly: +5 initiative to troops, +5 spell power for dark spells and -10% to their cost.
During the campaign, these will be very useful:
Elementalist's Ring (+20% summoned creatures)
Sar-Issa's Robe (-50% to the cost of all spells)
magister or prophet set
artifacts for +initiative, +spell power, +mana
And finally, a couple of tricks not yet known to everyone from our foreign colleagues.
#1. When clearing neutrals, we often need mana. It can be restored not only with wells but also by killing enemy stacks on the battlefield. 3 mana for each. A discovery for me was mass suicide of single units for mana restoration.
For every one of our lone ghouls sent to their death, we get 3 mana. Convenient, considering that for the Requiem, ghouls are just meat!
Plus, sometimes living neutrals join us. Well, dear reader, you've already understood me. Meat is meat, 3 mana for each!
#2. At level 1 we take Tactics, with which we place skeletons in one of the corners — and surround them with ghosts. Voilà, hero auto-attack + skeleton volleys — and we can clear almost any stacks of cavalry and infantry. It's all about the Ghosts' "spirit" ability = -50% incoming damage until the first action. And with tactics, they can perfectly stall for the whole battle — and cover the skeletons.
#3. With the purchase of vampires, it becomes possible to clear large neutrals using only bloodsuckers. Vampires in the corner, plus their "out of time" ability = the first attack misses. Add the Restoration skill — and you can clear various lamassus and riders of light without losses. Especially if enemies approach one by one. The main source of damage is the hero's attack. Slow, but effective.
They say Necromancers are OP. I don't know about everyone. About the Requiem — I'm sure.