Some general tips, and a bit of rant how the olde days were better.

Just a few tips and observations about the game, so less people would ignore some really good cards out of being weirded out by the initial information.

Intro/General Info

Currently there are two characters, Number 7, which is easy to play as and is the only one unlocked in the beginning, and dollmaster Maribella, which relies mostly on spells. Every time you play as specific character you gain some experience (dunno how it works so far) and may gain level. Each level provides some new cards, decks or relics you start the game with, or acquire through the game. If you're new, you might want to complete tutorial mission, and if you're not, you can complete the tutorial mission in under 90 seconds to get the achievement 'gotta go really fast'.

Considering that this is an intro, I will say the following:
You can damage structures if you aim your spells at them. Let's say you have a sliver of health on the village or castle left, if you use your spell, it will ignore all armor of said structure and deal direct damage to the building. There are exceptions, like finger of death.
Enemy has relics, and you should chekc what they do before engaging, so you won't be surprised by inability to use your cards at some points, or getting random card replaced with another.
Souls are tasty. Sometimes it's better to hoard them instead of spending.
Question mark sites are fun, and can give you extremely rare cards or relics, though some require certain cards/souls.
Always carry a squad card with at least 50 mana cost, you'll know why at some point. (For random encounter.)

Cards Overview

You have three types of cards: Squad, Spell, Utility.
All cards have some information on them about what they do, for example Squad cards show you the type of unit you will summon, how much damage it deals per second, it's health, amount of units in one group, armor, speed.
Note that if enemy has certain relics, you can have your cards 'chained', and will have to either lower your total mana or redraw them.

The Squad cards can be played anywhere on the field, and they will always spawn a specific squad at the portal, as long as you have enough mana.In case of Skeletons, you need 20 mana to summon a pack of them, and they provide no additional benefits.

The Spell cards once drawn from the hand will create a circle, in which they will be activated to buff, debuff, damage or heal untis.
With case of Fireball, you will have an area of effect where you will be able to damage enemy units. Note that you will not damage your own units, but you can damage enemy buildings as well.

The Utility cards often provide some form of, well, utility.
The overdraw, for one, will allow you to draw one card, or two, even if you are at your maximum capacity.

Now the cards themselves may have certain additional effects. For example:
Demonic strength is a utility card with Perk and Exhaust tags. There are also Overdraw and Destroy tags. When the card with Exhaust tag is played, it will be unavailable until the end of the combat. Card with Destroy tag will be removed from the deck entirely, but that's a given. Perktags work until the end of the end of a combat, which you can see on the bottom right of your screen. Overdraw works for as long as you are exceeding the maximum card limit in your hand.
In case of Demonic Strength, when played it will increase the damage dealt by your units for all the next played Squad cards.
With the tactical overload, you will have increased mana regeneration for as long as your hand stays above maximum capacity.

Relics

Relics are passive buffs that stay with you from one battle to another. You gain relics from elite and boss battles, as well as random encounters. There are quite a lot of relics, and you should read about them in necronomicon.

What relics to use? Good question. I can't say for sure which ones you should use, or which one you will get, as they tend to be randomized. But I always try to get myself soul jar, a possible reward from bat, skeleton bell and enchanted pillar. The first one gives 100 souls every time you upgrade a card, the second summons 5 skeletons every time you cast a spell, and enchanted pillar gives 25% faster mana regeneration at the cost of lowering your portal health by 25%.

There are a few relics that you always can get from certain encounters:
Red gemsstone: +25% unit attack. Can be gained in a random encounter with a cat burglar, if you have a squad with 50+ mana cost in your deck.
Blue gemstone: +50% unit health. Can be gained from a statue. If you don't want to lose health, bring squad with 50+ mana cost.
Fishy fish: no clue what it does. Get it by replaying the one of the evenets with red or blue stone.
Screaming Breastplate: +20% unit health and attack. Get it by paying 150 souls to a soul mage in random encounter.

Relics are supposed to be bought in shops as well, but I never saw one on sale, unfortunately. Or they are extremely rare.

Death Knight

Number 7 is your starting character, the easiest one to play, but you will also lose with her the most, probably. I know I do.

Her main ability is that she will always draw more cards for free, as long as she stays under her hand size. The best of the two innate abilities in my opinion, as you don't need to spend time or mana waiting to redraw your cards.
She has two relics. Starting relic is Hellforged armaments, the most efficient relic in her deck, which provides +25% attack value to her units. The speed debuff of -10% isn't much of a tradeoff, you mostly want your units to stick together anyway.
Her second relic is heart of legion, provides +50% hp but increases mana cost of your squads by 5 mana points. I haven't played with it, but additional health can be acquired through other cards.
These two starting relics determine whether you should focus more on tanking or on assault, but they are not as powerful as some relics you may acquire later.

I would consider Number 7 a general type. She has cards which mostly empower her units and she also has access to some unique to her cards, like Orcs and Abominations (At least I think it's the case). You should always read through description of your troops, as when you play Gladiator card, you will also increase health of other cards you currently hold.

Then there are her utility cards. Some of which allow her to overdraw, which is useful for certain perks, or get the said perks.
Her undoubtedly best utility cards, even unupgraded, are Unleashed Strength/Vigor/Fury, as they provide every 2 seconds an increase to certain stat of all squads currently in hand. So it's entirely possible to have a squad with +1000% in health, though you can't play that squad for quite a while and can't redraw the deck/get it sealed off, or the bonus will be void. So, it's best to have one-two of each Unleashed card.
On the other hand, her Demonic strength is a one time use perk, which will make all units stronger from that point on. My favorite upgrade is Armored Demonice Strength, as you get free 2 points of armor, and considering that most units get up to 5, and more ofthen than not, 0, having 5 cards played will make your units last for much longer time than without it. Though you still have to play these cards.
Tactical overload is a must spell for her, as it allows for 25% faster mana regeneration while overdrawn, and if you have it upgraded to Powerful TO, you will get 50% increase. If I can, I carry two of those.
Phalanx and Strength in numbers are useful with relics that increase hand size, as you will need 3 or more squad cards to have the bonus activated, or 2 in case of phalanx. Overdrawn perks, aside from tactical overload, aren't that useful in my opinion.

Her Spells are mostly designed for the support, like with Blood rage which increases attack and movement speed, Armor up, which increases armor, but lowers movement speed, or arrow rain, which if upgraded lowers movement speed. I mostly use blood rage in combination with armor up and mark of titans to quickly destroy villages and castles.

Malevolent Wizzard

Maribella the Doll is mostly a magic caster, with special ability being cheaper redraw costs, or free if you use it after cooldown. She's hard to play in the beginning, due to low amount of spells and perks, but she will snowball after only a few battles, if you manage to get your hands on the right cards and relics.

Her starting relic is spirit skull, which increases her total mana by 25. It doesn't seem to increase her mana regeneration however.
The other relic she can have is cursed tome, which will lower the spell cost by 25% but increase utility cost by 25%. Both are actually quite good, but cursed tome seems more efficient so far.

She doesn't seem to have access to unique squad cards, instead she has tons of summons. For one, her bears, while not exactly powerful or cheap, are actually decent sort when paired with regular squads. Though, again, their cost is astronomical, being 80 mana for three bears with 400 health.
However, this is not the main card. Her main cards are the 'towers', which will attack enemies and can block enemy path, and 'factories' which will produce enormous amount of units, if played right.
Her signature attack spell is Wrath of Oggy, as one such spell will shave off 1K of health from every unit and building on the map, after which you can make a couple of haunted villages with extra meteorite. However, it's one use per battle spell, and should be used after enemy made a few units. Basically a cleanup card, though some enemies have slightly more than 1K of health. Or much more.
The Tomb and Toy Castle provide unending supply of minions every few seconds. Sort of. They are rather costly, but they are still cheaper comparing to summoning regular units and you can place them near enemies, though they can be destroyed. Also you can repair them with patches/healing spell and prolong their working hours, if you manage to get the healing cards before they cease production.

Her utilities aren't that great however, but Quick Incantation lowers the spell cost by 3/5 mana points, and it's somewhat common card, at least in my playthrough, where I managed to get about five of those. Rapid Evocation and Efficient Evocation are quite rare, but worth it, as you can redraw faster for free, or spend less mana per redraw.
The Delivery cards are useful, to a degree, but most of the time they become redundant after the first sector.

As to her main relics, they should be the Skeleton Bell and Necromancy for dummies, if you manage to get them. Skeleton bell summons five skeletons each time you use a spell card, and Necromancy for dummies spawn skeletons at haunted village when you redraw.

Some boomer rant

Back in my days, the necronator was a funny flash game where you went around as an evil, if noobish, necromancer, pillaging villages and hanging do gooders. Nowadays it's just some reanimated corpse going around playing cards with humans. Ah, the days when one could just send one zombie on a manslaughter (and woman), and chill with some cold beverage in hand while observing the massacre.
Good old days. You can still relieve them until this december. The original Necronator should still be available on Armorgames or Newgrounds. Didn't like the second one though, don't remember why. Would be nice to have it on steam to play after the soon to be bygone age.

[Dr. Bloodraven]

The original necronator and infectonator were great.

[Ethinil]

Thanks for the guide !
I never sawn the tag Destroy. It seems it doesn't exists anymore.

And for the ol'good Necronator, Pepperidge Farm Remembers ! ;)