Tainted Grail: Conquest

The Necromancer is the hardest of all 9 classes to play in my opinion, as it has the most complex set of mechanics to learn. Necromancers are part of the Moonring faction, so fight like the Summoner and Blood Mage through the use of minions, though the Necromancer can also fight directly. Necromancer minions by default are summoned at level 2 and each turn will activate, lose one level and deal damage to the Necromancer for each level remaining (if not Sacrificed). However, the Necromancer can Sacrifice these minions and they will become an Ethereal version, which have different effects and are vastly superior to the living version (except for the Golem). The Ethereal minions cannot be Promoted or buffed anymore, so whatever level they are when they are Sacrificed is how many turns they will reamin on the battlefield (unless you Sacrifice them). Ethereal minions do not have health, so they are not targeted by enemies, which is why you do not Sacrifice Golems (or have one Ethereal and one Living). The Necromancer Ultimate is unlike any other, as it transforms the Necromancer into a Lich. When you transform, any minions you have that are Living will be Sacrificed and become their Ethereal form. The Lich has its own deck of cards, which will be discussed later. It takes 6 Charges to transform and you cannot use it with any less, with Charges being gained whenever a minion dies, is Sacrificed or an enemy is killed. Each kill wil also award you a 0 Energy Summon of a Ghoul, which deal a small amount of damage to all enemies and last 2 turns unless they are killed. Ghouls cannot be promoted, but can benefit from buffs.
The Necromancer minions are:

Golem:
Living - Attracts all attacks towards itself. Activating the Golem will heal it for the amount shown above its head.
Ethereal - Place a Barrier on allies.

Fae:
Living - Heal the Necromancer for the amount above the head of the Fae
Ethereal - Gives you 1 Ultimate charge per activation and 1 extra Energy while it is on the battlefield.

Abomination:
Living - Deals a single attack to all enemies.
Ethereal - Deals a single more powerful attack to all enemies. Each attack it makes raises its damage by 25%.

Wyrm:
Living - Deals 2 attacks randomly.
Ethereal - Deals 3 more powerful attacks that also remove 5 Armour randomly.

The Lich:
Once you have a fully charged Ultimate, wait until you have a full set of Energy before using it. This is the one thing that is shared between the Necromancer and Lich, though transforming will give you +1 Energy even if you had none, if the enemy is a single attack from death this can make sense to use. The Lich has a base deck of 9 cards, most of which you will never use. The cards are below, ranked in order of usefulness.

Power Steal(1) - Lower enemy attack by 50% and increase your own by 100%. This damage buff affects minion attacks too, which is what makes it the best of the bunch. Play this after you play Duplicate for a 200% buff and 100% reduction.
Duplicate(1) - Double the effect of the next card played. Always the first card to play when you become the Lich, as it allows your Power Steal to be twice as effective.
Eerie Gaze(1) - Reduce enemy Armour by 75 for the rest of combat. This debuff can make tough enemies much weaker and can be played each time you transform
New Beginning(1) - Discard remaining cards. Next turn when you turn back, draw as many cards as you discarded with their cost reduced by 1. Play this if you need more minions or want to activate a lot of times next round or need to regenerate Barrier as well as playing offensively.
Energy Wave(1) - Deal 1 attack to all enemies, gain 10 Barrier per enemy hit. The best damage dealer available, as it also helps you not die,as the Lich is rather fragile.
Tongue of Fire(x) - Deal one hit per Energy point to a single target. If you don't care about anything else except raw damage right now, play this.
Hell Rays(1) - Deal 3 hits to random enemies. Against a single target, play this as well as/instead of Tongue of Fire. Against more than one, you can pray or just take the damage if you don't care where it goes.
Funeral Fog(1) - Gain 50 Barrier and 2 Blocks. This is very situational. Play this only if you are facing overwhelming enemy attacks and must play this or die.
Shocking Grasp(2) - Deal a hit and Stun an enemy for 2 turns. It is always better to deal more damage to the target than Stun it, unless you need to reset and attack/damage buff and have no other options.

To be successful as a Necromancer you will need to do a few things. First, always have a Golem to protect you from damage, but do not take this as your automatic summon. That should be Wyrm, as you need to deal consisent damage. A Necromancer should always be summoning more minions, you don't care whether it is Wyrm or Abomination, you need the damage output. The cards that summon minions at Level 4 are very valuable, take them whenever you see them. Also crucial is Sacrificing minions at every opportunity if they are Living and not a Golem or you need to heal with Fae. If there isn't an improved summon card, take a card with Sacrifice or that allows minions to be activated. You should also keep a small Barrier around the Necromancer, to soak up the damage from Living minions (or stray attacks after a Golem is slain). Unlike the Blood Mage, the Abomination and the Wyrm both are useful to the Necromancer, so take Passive Skills for them both. Abominations can achieve more power thanks to their damage scaling, so can be better than Wyrms in some cases, though the Armour reduction from the Wyrms is also very useful. An absolutely critical Passive Skill is the one that gives you 2 Ultimate charges per Sacrifice, so grab this the second you see it. You want to become the Lich as much as possible, then use Duplicate/Power Steal to give you 300% damage for a turn. Use the rest of the Lich's energy as you see best. Try and go Lich when you have at least 1, but preferably 2 or 3 damage dealing minions around.

The priority for combat is as follows:
Summon Wyrm/Abomination
Sacrifice Wyrm/Abomination
Summon Golem if needed
Place Barrier on Necromancer
Buff Living minions

If you follow this formula and fight every battle you see, you should reach level 20 in the third area, just before you fight the 3rd guardian.