Via BOLS:
Tyranids will get a massive overhaul with Psychic powers. Look for many new non-overt force powers that represent the chilling, horrific, warp-stifling power of the Hive-mind.
Aside from a few old chesnuts, look for almost all new Tyranid enhancing abilities, and other new chilling and damping abilities to use on enemies of reflect the horror, hopelessness, and dread of fighting the swarms under the psychic interference of the hivemind.
Look for concepts such as dampened psychics for opponents mixed with bio-enhancement powers to bolster the swarms.
Shadow in the Warp will remain in the codex, but receive a controversial overhaul.
More existing units will have the option of manifesting Psychic powers.
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If you missed the earlier post:
http://40kwarzone.blogspot.com.au/2013/08/rumour-watch-tyranid-rules.html
Cheers.