Cryx serve the biggest dragon of them all, Toruk, empowering their technologies with decay and necromancy. They outlast you, weaken your defenses, and bring you down with terrible magics. Some of their attacks feature Corrosion Effects and Corrosion Damage.
One way to start Cryx is by buying the Battlebox and playing small games using only a warcaster and the four warjacks that come with her.
What to Order
Two Battleboxes were produced for Cryx. The first was all metal, and you can tell it's the older box because Warwitch Deneghra appears like this:
Product Code: PIP 23001
However, the plastic boxed set has the following product code:
Product Code: PIP 34067
Deneghra is, to many, the archetypal Cryx 'caster. She's quite versatile, and her spell list (And her awesome feat!) is full of Debuffs to help her army get to where it needs to be. Crippling Grasp can hamstring your opponent's big hitter until you can deal with it later, and Parasite is great for leveling the playing field for your units. In addition, her Ghost Walk spell is vital if you're playing a unit that likes to move through terrain/you want to move through terrain.
The Slayer is the generic Cryx heavy, but generic hardly makes it sub-par. While it does tend to require the full 3 FOC to operate at it's height (Though you can get away with allocating 2), when it's properly supported with things like Parasite or Scything Touch it turns from a humble 'Jack to a monstrous damage inflicter, capable of wrecking 'Jacks in a turn, though be careful, as it's quite fragile for a heavy.
Deathripper Bonejack x 2
The Deathripper is an odd-duck in Cryx. It's a melee oriented 'Jack that's sporting an Arc Node, thus many people prefer the Nightwretch. However, the Deathripper is great for slamming enemy models, and they're one of the cheapest light 'Jacks in the game with an Arc Node. I personally get great use out of them, and I've even had them assassinate 'casters when necessary, though this shouldn't be a standard tactic.
The Defiler is a Bonejack with a nice ranged attack, it's corrosive spray can really lay out the hurt if it can hit. DEF debuffs on enemy targets will definitely help the Defiler come out on its own, and despite being one point more expensive than the Deathripper I always try to include at least one in my lists.
Tactics with the Cryx Battlebox
Cryx as an army focuses on speed and magical attacks/spells, and the Cryx battlebox is a great example of this. While most of the Battleboxes have at least 1 Arc Node, note that the Cryx battlebox has 3. This gives you a large advantage over your opponents, as you'll be able to sling spells from far away with Deneghra while your Arc Node Bonejecks move up to deal with the enemy. Something to keep in mind is that while most of the heavies in the other faction Battleboxes sport high armor values, you'll notice that the Slayer's armor and damage boxes reflect something similar to a 'light 'Jack, but don't despair! The Slayer, with the right help, can smash around almost any other heavy out there, though it'll cost a fair amount of focus. In particular, Deneghra's Parasite spell is fantastic in combination with the Slayer, if you cast Parasite on something a turn before you want to charge it your Slayer, you can allocate the full 3 FOC necessary to beat the Parasited model to scrap. In addition, while the Deathrippers are normally best left out of combat (because their arcnodes will be useless if they're engaged), if you can get Crippling Grasp on a target and charge both Deathrippers at the target you're guaranteed to at least mildly distract your opponent, and if your up against one of the really 'light 'Jacks like the Charger or Repenter you may even destroy it though.
Do note that models affected by Ghost Walk can ignore free strikes. Feel free to cast it on a model in combat and have them leave combat, possibly to reach out and touch your enemy Warcaster.
However, Deneghra's main tools of assassination in the Battlebox are the Defiler and herself. If you can position the Defiler within the vital 8" (Doing this involves a number of strategies, mainly engaging things with your Slayer and/or one Deathripper, try to keep at least 2 Arcnodes unengaged though), and pop your feat, your opponent's caster will be vulnerable to Deneghra's Venom spell and the spray attack of the Defiler (You may or may not want to allocate 2 FOC to this 'jack, but remember that the more FOC you allocate to it the less FOC you'll have for Denny's spells.) Don't be afraid to sacrifice one Deathripper or so for victory.
Beyond the Battlebox
Cryx has tons of unit options beyond the Battlebox, and Deneghra in particular has a great synergy with many due to her versatile debuffs. Bane Thralls are a great unit to add in, they'll not only give you a rock-solid infantry unit but their Dark Shroud rule will keep the debuffs going. Mechanithralls are also great, they're cheap distraction models that can tear 'Jacks and heavily armored infantry to pieces if they get into combat. Finally, solos like the Skarlock and Warwitch Siren are great for giving Denny the max amount of FOC possible, while solos like the Pistol Wraith can hamper your opponent's movements.