Seriously GW, don't you think that you have assraped the C'tan enough already?
The Tesseract Vault is a Necron machine which contains the essence and power of a Transcendant C'tan. But at the same time it focuses the C’tan’s powers so that a C’tan enchained in a Vault is more powerful than one outside of a Vault.
In defense of the poor power-stripped C'tan I'll say that he/she doesn't like that (and who would?) and he uses every bit of power left within him to unshackle him/her/itself by destroying the Vault bit by bit in the process... if it weren't for the legions of Canoptek Spiders and Scarabs who repair it continually, rendering his/her/its efforts irrelevant and useless.
Basically, there's two versions of the Tesseract Vault you can choose from. You can have the AV 14 floating recliner for your C'tan shard, or you can be ballsy and split the two. With the latter option, the Tesseract vault transforms into the Obelisk. This gives you a bad ass monolith that auto hits flyers.
7th Well, now both of these things are mentioned in the codex.
The Vault has now gotten a "boost..." Keeps the Apocalypse aspect of lots of Hull points to take hits (AV 14 all round, as Necrons like it). This thing is just an upgraded C'tan, now that may not sound that impressive at first glance, but when you look at the upgrades it's rather fitting.
This thing is meant to be sent in and explode in the face of the enemy, as a big shooty floating distraction from the rest of your army. It even got a specific rule that says that it will explode. So let it do it.
Still pretty big hovering thing that will (have a chance of) deal out some damage no matter what you shoot at.
Should be noted that since escalation is still sold the old rules are still legal.
The Vault got quite the power boost. For 500 points (a cost beaten only by the Seraptek) you get a Lord of War unit with 24 wounds, a 4++ save thanks to Transdimensional Force Field, 4 Tesla Spheres (which are R24”, Assault 4, S7, AP0, D1- but a natural hit roll of 6 scores THREE hits instead of one. Did I mention the Vault has four of these, meaning there are 16 shots total?), and it can (assuming the Vault has more than 16 wounds left) cast three Powers of the C’tan per turn. It serves four purposes, some of which it does better than others- it can cast Powers of the C’tan, it’s quite tanky thanks to its 4++ and (if you’re Szarekhan) a 5+++ FNP, it’s an absolute middle finger to troops, like Ork Boyz and Guardsmen, and should it be destroyed, it serves its final purpose- a tactical nuke. For 3 CP (it’s normally 1, but the Vault is TITANIC) the Vault automatically explodes- everything within 2d6” eats d6 mortal wounds.
- Apocalypse - This is the game mode where you can play this... thing.