I have taken a little break from Genestealer Cult, but as I take another look at them, it hit me that a very solid ally for their faction would be an Astra Militarum Lord of War, such as one of the Baneblade variants. Specifically, as I reviewed the different kinds available, the Stormsword jumped out to me as one of the most competitive super-heavy tanks and one that would work quite well with the Genestealer Cult units/tactics.
Looking more closely at the Stormsword, we have the following:
-Super-Heavy vehicle — This means that, among other things, it can shoot all its weapons every turn and can fire them each at a different target, if you wish (and it can do all this even after it moves up to 12″). It also means that it Ignores all Crew Shaken/Crew Stunned/Immobilized/Weapons Destroyed results and that Explodes results do not destroy the Stormsword outright, but instead inflict D3 additional lost HPs on it.
-Stats/Upgrades — With BS3, AV14/13/12 and 9HPs, the Stormsword is quite survivable and generally takes massed melta weapons and/or massed dedicated anti-tank attacks to bring down. Needless to say, combined with the Super-Heavy rule, this makes it an ideal “fire magnet” type unit to soak up enemy combat power away from the rest of your army (more on that later, as it relates to Genestealer Cult allies). In terms of upgrades, you can add a H/K Missile, a SB/Heavy Stubber, and up to four Lascannon/TLed Heavy Bolter Sponsons (which are purchased two at a time for 10 MBs each). Given the ability to independently target every one of these weapons, and combined with the stock TLed Heavy Bolter and Stormsword Siege Cannon, if you take every upgrade possible, the Stormsword can shoot at 11! different targets each shooting phase (and 12 targets for one shooting phase, if you use the Hunter-Killer missile).
-Stormsword Siege Cannon — Outside of the incredible durability/”tanking” ability (literally 😉 ) of the Stormsword, this is why you are taking this Super-Heavy. This weapon has a range of 36″ (so a threat radius of 48″, with the 12″ move of the vehicle included) and puts out a single S10 AP1 Primary Weapon, Apocalyptic Blast. Ignore Cover shot each shooting phase. Translated into English, that is basically a Railgun Shot with 10″ diameter blast that ignores all saves except Invul Saves, causes ID to anything T5 and below, and Penetrates AV14 armor on roll of 5+ (with re-rolls for failed penetrations, due to being a Primary Weapon). Outside of Destroyer Weapons, this is one of the most powerful shooting attacks in the game and can be used every turn (unlike, say, a Deathstrike Missile).
So, how does all this relate to being an allied force-multiplier for a Genestealer Cult main army? Well, first of all, to access a Stormsword, you have to take an Astra Militarum Combined Arms detachment (for the LOW slot), which means you have to take at least one HQ and two Troops as well. Combine with the nearly 500 points for the Stormsword and you have a lot of points invested before you can even begin making your main Genestealer Cult army… so, this begs the question, why do it? Really two reasons, I think, but they are very solid reasons:
 Firepower — On its own, the Genestealer Cult faction has some decent firepower, but it largely short-ranged and there is really nothing that can just devastate entire units in the shooting phase (Genestealer Cults are strong on terms of mobility, Psychic powers, and assault to make up for this). An allied Stormsword changes that whole equation… with such potent firepower built into a 10″ Ignore Cover blast, Genestealer Cult players can use it to deal with enemy Deathstars, hordes, massed vehicles, MEQs in cover, MSU Jetbikes, you name. About the only target that the Stormsword has trouble with is MCs/GMCs with T6 or above (and multiple Lascannon Sponsons can help with that)… with all their Rending close combat attacks and the ability to assault from Reserve with the right rolls, Genestealer Cults are already very well equipped to deal with these opponents, so the Stormsword Siege Cannon becomes to a them a force-multiplier against all other targets.
 “Tanking”/Distraction ability — More than anything, this is where I think a Stormsurge brings advantages to a Genestealer Cult army. Both in terms of actual combat power and psychologically, a Super-Heavy vehicle like this, especially one that cannot be ignored because of its amazing firepower, can “pull” focus away from the relatively fragile Genestealer Cult infantry/vehicles and allow them to operate with little to no impedance, thus maximizing their abilities. Put another way, the “anvil” nature of the Stormsword compliments the “glass cannon” nature of the Genestealer Cult and, used properly, both can make the other better and the army more effective as a whole. Now, while it is true that a lot of the firepower that is strong against Genestealer Cult units (such as small arms shooting, flamers, blast weapons, etc.) will not be effective against a Stormsword, and so the Super-Heavy will not be able to “pull” that kind of shooting away from its Genestealer Cult allies, because the Stormsword is such a threat, it will in many cases force your opponent to drastically alter his normal battle-plan against the traditionally light infantry Genestealer Cult army, which can create opening for your forces to exploit.
 Synergy Abilities — Finally, there are several ways in which Genestealer Cult and the Stormsword can synergize. The Lord of War can act as a physical shield for Genestealer Cult troops with its massive bulk, protecting them from enemy shooting attacks. Conversely, cheap Acolyte Hybrids and Neophyte Hybrids can make an excellent “bubble wrap” for the Stormsword, protecting from enemy assault and also preventing “meltacide” units from physically getting clos enough to it to destroy it with massed melta shooting. Additionally, Genestealer Cult psychic powers, namely maledictions from the Telepathy and Biomancy disciplines, can enhance the Stormsword (e.g. if you can Enfeeble on a T6 enemy MC, like a Riptide, then hit it with the S10 Stormsword Siege Cannon, if it fails its Invul saves it will be instant-killed). Finally, the Stormsword can use the Thunderblitz table, in conjunction with the hyper-powered tank shock/ramming capacity of the Goliath Rockgrinders, to actually do very significant damage to enemy forces via “melee” attacks, which gives you an alternate option for how to inflict lots of damage (note: the Rockgrinders can use the Super-Heavy for cover as they advance, until they get within Tank Shock range).