Playing as Necromancer

The Necromancer looks weird and weak on first glance.
The first one is true, the second one quickly turns as far from the truth as it can be.

In the beginning the Necro is indeed rather weak. His Cities produce ghouled versions of normal units, that are slightly more vulnerable and do not regenerate on their own.
But soon this weakness turns into a strength and I'll show you how.

In the beginning your cities grow slower and ghouls are weaker than everyone elses regular units.
Your Undeads don't regenerate on their own and have -1 Defense and 20% Spirit weakness.

The first step to counter this, is to build two of your class specific buildings in your cities as soon as possible. The Harvesters Guild to make your city grow faster and the Embalmers Guild thereafter, which also accelerates city growth a little, but much more important, it gives all your ghouls +7 Life, 40% Fire protection and heals them by 50% of their (un-)life when stationed in the city. This makes every ghoul unit completely restored after staying in the city for two turns.

After that and the necessary defenses you should focus your cities on research, because all the real power, the necromancer has to offer, comes from researched empire upgrades and global spells.

You should try to get those researches first, but don't stress it, if one or more aren't there directly because of the random appearance, just take another one - all of those researches will appear at one point:
Shrines of Unlife - Shrines boost Mana output and give 50 undead Population per turn (60 research)
Master of Puppets - Gives your ghouls +300 Morale (180 research)
Enemy of the Faith - All Units get Devout Slayer and Support Slayer (80 research)
Power Ritual - All your undead units get +2 normal Damage and +2 Blight Damage (200 research)
If you've got none of them to choose at one point, there's always room for a combat spell, a summon or Seafaring I or II.
Enemy of the Faith and Power Ritual are global Spells with 20 and 40 Mana per Turn upkeep. In my experience this is not a big deal, but make sure you have the mana income to sustain them.

The Focus on your Cities should be to continue to get more research. Once this is done concentrate on population income and production. Don't neglect defense in between - I prefer 4 ranged units of diverse damage types, if possible, and 2 Melee Units with good defense to block the gates or throw back flying attackers. In the beginning a T1 Infantry unit with a shield should do, but as soon as the invasion Events start happening, get something with more punch.
Also, if you have the two spells active from the early game, keep them up and get even more Mana, you'll need another 80 more per Turn soon.

Research wise, you should instantly research Age of Death as soon as it shows up in your research book, even it costs a lot of research points - 2400 to be exact. Once this bad boy is researched, instantly start casting it.
Once you've got that spell active, after every battle each army who lost units in the fight has a 35% (that's more than 1/3!) chance to be ressurected as ghouls under your command. And not only battles where you are participant. EVERY BATTLE. The only units who do not rise as ghouls are Leaders, Heroes, Elementals, Incorporeal units, Machines and undead ones. But you'll get already undead ones with other strategies, see further down the Section about Necromancer Heroes.
You'll quickly have a huge number of new units all over the map. Keep only the ones you really like and the really strong stuff and even then, you'll most probably quickly run into trouble to pay them all. So this is some management to do, but it really makes you really powerful really quick.

This spell instantly fires you into the late game.
If it doesn't show up on your research list, get the other empire upgrades in the meantime.
Vampiric hunger - gives all your undead Infantry, Irregular and Pikemen the Life Stealing ability (240RP)
Healers of the Dead - gives your support units Heal Undead and Reanimators get Lesser reanimate Undead (400RP)
Harbingers of Death - Your Leader and your Heroes all are now undead, have Night Vision and become Archliches, which cause all your units starting their turn in this heroes army turn to ghouls. Since your Ghoul/Undead units should be severely buffed by now, that is something desireable (400RP)

Get as many Necromancer heroes as you can, since they can learn Inflict Ghoul Curse at Level 9 - This turns any attacked living unit failing to resist (except Dragons, undead...) into ghouls under your command after the battle.
They can also learn Necromantic Aura at Level 11, which causes all undead units attacking the hero and failing to resist to switch sides and come under your command.
At level 5 already they can learn Control Undead which is just like Charm or Convert for undead enemies.

Late game is easy going for the Necromancer.
You're getting new units left and right. The more enemies are warring on each other, exploring guarded sites and are generally fighting, the more new units you get, since more than every third army who loses a battle, rises up under your control.

City wise, go for the stuff that brings mana and money and then finish the city with all the other stuff left to build and get new cities. Dont bother with diplomacy on neutral cities and dwellings, just overrun them, rinse and repeat.
At that point that your armys get big enough, choose your favorite enemy player and throw your hordes at him. 1/3 of his fallen armies will rise under your control too.

If your armies get too big to be sustainable by your income, dispand some until your income is balanced again and then end Age of Death so you don't get new units all the time. Once your treasury recovers re-cast it.

Research wise, go for the remaining empire upgrades and global spells you've not gotten so far first, then Seafaring and Logistics then combat and city spells and remaining Necromancy levels. Take summons last. You'll not need to worry about units at this point.

The other players will need some really good strategies and great units to stand against the mountains of units you can throw at them at this point. The ones with the most (even still small) chances are Dreadnoughts, whoose machine units are immune to your necromantic powers and Theocrats with all their spirit damage, that a Necromancer never gets really good against.

Give 'em hell, or worse: Undeath.

Heroes
Depending on how many Heroes can be under your control, don't get every single one offering to join for two reasons. First, not all Heroes are useful to a Necromancer. The Convert Ability of a Theocrat might be nice, but you've got plenty of ways to get new units as a Necro and all his healing and morale abilities won't work on your undead armies. Same goes for the Dreadnought. His ability to command and improve machines is something the necro simply doesn't need.
The Arch Druid is okay to use, his resistance abilities work quite well with your hordes and he's a good ranged damage dealer. Sorcerers are great magical damage dealers and have a wide array of good abilities to chose from. Rouges and Warlords are both fine as good Melee units with tons of buffs to get.
The second reason to reject a hero is, you can attack him. Killing a hero that wanted to join gives you a nice drop in alignment, and that's what you want, especially if you've chosen the shadowborn School.

Alignment
Not too much to say here. A Necromancer goes for Evil. Especially a Shadowborn Necro. If you did not take Shadowborn, it is in theory possible to play a good aligned necromancer. Weird, and not in harmony with Age of Death, since you'll have to spare fleeing guards, but entirely possible.

Races
The Necro doesn't care. He works well with every race. But there are a few races that work even better, since their inherent Bonuses counter the initial weaknesses of the Ghouls or are otherwise especially useful.
Dwarves get +1 Defense and cancel out the -1 Defense of Ghouls.
High Elves get +3 research per city which helps getting to Age of Death a little quicker.
Orc units have +5 life, countering the lowered Defense and Spirit resistance somewhat.

Lairs
If you are far away from other players, try to keep some lairs unexplored until Age of Death is active. The often powerful guardians may rise as your ghoul servants then.

Bolstering newly acquired Cities
If you got the mana to spare and the summons already researched, it's a good idea to cast a banshee or even a dread reaper into the garrison of a newly conquered, founded or bought city.
It takes quite some turns to build all the structures for afterwards building fully boosted defense units. If you don't want inferior defense units, or keep your attacking army standing there and wait until your defense rose up, a dread reaper (or another powerful summon) can defend the city against wandering bands of brigands and monsters.

Farms, Springs of Life and Wells of Souls
Do not forget to use "Corrupt the source". Normal Farms and Springs of Life don't contribute to your city growth until you used that spell on them. It's quick and cheap.
Wells of Souls are your special friend playing as Necromancer. They allow you to heal completely and they turn all living units into ghouls when there is no Hero nearby or Harbingers of Death isn't active, yet. Within a cities domain they give 100 undead population and 10 mana per turn.