There is someone over on Warseer that claims to have the codex, and I it looks iffy on whether or not this is true or not. For now though, we will just report on what is being said about the codex there.
Please remember that these are rumors.
Sinsigel has very much proven that he has the codex. Check here for more information
http://natfka.blogspot.com/2014/01/with-tyranid-codex-in-hand.html
via Sinsigel over on Warseer
I bought the new tyranid codex to share it with my fellow gamers yesterday.
Pyrovore remains largley unchanged save that it gets slight boost to wounds, attacks and initiative.
Its cost is slightly reduced while acid maw attack is given at a bit higher strength but at the cost of making only single attack.
Mawlocs and trygon both are cheaper. Mawloc especially is about 20~25% cheaper.
Mawloc's special rule Terror from the Deep is still much same......except that when the damage is resolved hiding in ruin or cover of other kind is literally useless.
Mawloc can also resolve damage(via Terror from the Deep) more than once if there isn't enough space.
Shadow in the Warp gives penalty to nearby enemy psykers' leadership.
Harpy is also cheaper. Its wounds and attacks are slightly boosted.
Carnifex and tyrannofex both became considerably cheaper. Around 25~30 percent or so.
Carnifex can still take devourer with brainleech worms which remains same as before.(Just without its penalty to enemy leadership)
It has less attacks and slightly higher initiative, but it can inflict more than one Hammer of Wrath hits.
Both types of gaunts became slightly cheaper albeit the upgrades are bit more expensive.
Zoanthropes are also cheaper.
And as forementioned in my previous replies, impaler cannons will be the bane of eldar wave serpents if they come within range.
As for biomorphs, crushing claws are changed in a way that they don't give extra attacks but are more effective against vehicles plus strength buff.
Regeneration probability became much higher.
Tentaclids and electroshock grubs are effective against vehicles(Bionic EMP I suppose)
Other than some boosted version of already existing shooting attack, I see no significant difference.
Taking account of its crushing claws, Haruspex has decent strength(stronger than mawloc, weaker than carnifex) with small number of attacks.
It however can increase its attacks depending on how many wounds it suffered, while at the same time can regain the wounds.
Its blood is also acid while its tongue attack can pierce terminator armour.
Boneswords inflict Instant Death depending on to-wound roll while lash whip boosts wielder's initiative.
Warlord traits include buff to synapse range, reducing cover, turning a forest into more carnivorous one, etc.
Psychic powers include another buff to synapse range, FNP, pinning test with additional leadership modifier, pseudo-psychic shriek etc.
Last thing. Malantai is NOT destroyed by a single zoanthrope.
And no more steel rain for tyranids.