So, it looks like several parts of the incoming Genestealer Cult codex have been reliably leaked so far, including what psychic powers this faction has access to (for this review, I am assuming that they will be Allies of Convenience, not Battle Brothers, with Tyranids, so the Genestealer Cult Blessing-type powers cannot be used on allied Tyranid units).
First of all, Genestealers will have access to both Biomancy and Telepathy, easily two of the best “basic” disciplines in the game. We will see when the exact stats come out, but I expect a Patriarch rocking Biomancy (Endurance, Iron Arm, Warp Speed, Enfeeble, witchfires, you really can’t go wrong here with him) and a Magos with Telepathy (trying hard to get Invisibility and our The Shrouding to cast on other Genestealer Cult units, but if nothing else Psychic Shriek is an excellent anti-MC tool). Some of these powers will even indirectly benefit allied Tyranids, such as the ability to reduce enemy S/T by -1 (imagine a Hive Tyrant using its S6 attacks on T3 Marines, or a charging Carnifex deploying its S10 attacks against a T5 Nemesis Dreadknight or T5 Riptide, for example), and other powers will combine very well with the new “Cult Ambush” rules to make Genestealer Cult infantry into fearsome assault units (imagine Endurance cast on a Brood of 20 Purestrain Genestealers, all of which already have a 5++ and 3A base, according to the new rumors).
Beyond Biomancy and Telepathy, however, the Genestealer Cults get access to the new Broodmind discipline:
P — [Primaris] WC1 — Reduce enemy unit WS/BS/I/A by -1 — Works great against everything from MEQs to high octane assault units, making the Genestealer Cult units (and their Tyranid allies), much more effective in close combat and even more survivable against shooting, thanks to the reduction in enemy BS by this power. Double-tap an enemy unit with this AND Paroxysm and they will likely be shooting and fighting at BS2/WS2 (or worse) and so greatly reduced in their combat effectiveness.
1 — [Psychic Stimulus] WC1 — Friendly unit gains Relentless, Fleet, and can Run/Charge in the same turn — This is an AMAZING power (especially for only 1 WC) for any assault based army (Blood Angels and Space Wolves would kill to have it) and allows Genestealer Cult models to no only Rapid Fire/shoot Heavy weapons and still charge, but makes their charge range significantly longer and more reliable. Throw in the “Cult Ambush” abilities and you have all kinds of options of getting your army into combat reliably.
2 — [Might from Beyond] WC1 — Friendly unit gains +1S and Rage — Another amazing power, especially when compared to, say, Unleash Rage from the Sanguinary discipline. Again, this follows the theme of turning mediocre assault units (like Neophytes) into respectable combatants and making strong close combat units (like Purestrain Genestealers) into ridiculous blenders on a level comparable to Wulfen, Death Company, TH/SS Terminators, etc.
3 — [Psionic Blast] WC1 — 24″ S5 AP3 Small Blast– A very decent witchfire ability which provides ant-MEQ capacity to a Genestealer Cult army and works well in conjunction with other Witchfires, such as Life Drain or Smite (from Biomancy).
4 — [Mental Onslaught] WC2 — Focused Witchfire that has a roll-off between the caster and target, adding in their Leadership values. If the target wins, nothing happens; if they tie, the target has -3I; if the caster wins, then the target suffers as many wounds as he lost the roll-off by, with no armor or cover saves — A very solid “assassination” ability, which combines especially well with the Tyranid Shadow in the Warp when targeting enemy Psykers (i.e. -3 to their Ld) and which can combine with Psychic Shriek and/or Psychic Scream (from Powers of the Hive Mind) for massive damage, even to MCs/GMCs.
5 — [Mind Control] WC2 — If passed, a single non-vehicle enemy unit (that is not locked in combat) comes under your control and you get to shoot with it immediately during the Psychic Phase — Obviously, for a WC2 power, this is tremendous, allowing you to take a very powerful (non-vehicle, so no Imperial Knights or Baneblades, unfortunately) enemy unit and hit his own force with it. Whether that is SM Grav Cannon Centurions, a Tau Stormsurge, Eldar Scatter Laser Jetbikes, Necron Destroyers, Deathwatch Veterans with Frag Cannons, or even a maxed out unit of Ork Lootas, this can obviously be a game-changing ability for both Genestealer Cults and their Tyranid allies.
6 — [Telepathic Summoning] WC2/3 — If using 2 WCs, then summon either 5 Acolytes, 5 Metamorphs, or 10 Neophytes; if using 3 WCs, then you can choose to summon either 10 Acolytes, 10 Metamorphs, 20 Neophytes, 4 Aberrants, or 8 Purestrain Genestealers (can take any upgrades they have access to for FREE) — Like Daemon summoning from the Malefic discipline, but perhaps better, especially with solid reliability and the option to go for either 2 WCs or 3 WCs, based on your in game requirements. Combine with free Termagaunts from Tyranid Tervigons and you really can create in the game the “endless swarm” described in the fluff, while still having plenty of points left in your army for higher-end units as well.