The problem is getting to the point where my Rogue Trader would know that the Eldar are an option. What we are in the process of doing is we have headed over to the Koronus expanse and found a heretical Adeptus Mechanicus sect that has sent us to retrieve an item they believe will be effective against the Necrons. We are in the process of retrieving said weapon.

Meanwhile we have discovered through our spy network (because the Crusade has not been very forthcoming with information even though it was my character that called the Crusade in the first place) that they are engaged with multiple Tomb Worlds and one of them is actually the home of a Necron Guildhouse.

Wait, the Dark Mechanicum knows about your necron situation?

If your DM knows his stuff, expect an inbound chaos armada to mop up the winner. I'd strongly suggest pre-empting that with a direct alliance, and pretending you're completely ignorant to the fact that they're heretics.

And while Exterminatus is "viable", I would point out that against a necron world, it is unlikely to work. If any races have countermeasures to the Imperial Exterminatus arsenal, it's Crons and Eldar.

Exterminatus against a necron world can work if you use multiple vortex bombs.

I remember in one bit of obscure lore what they did was 'hey, use all of our lances to bore a hole right through the crust/mantel of the planet. Then drop a vortex torpedo into it' (this wasn't against necrons specifically, it's just a method of exterminatus available to those without exterminatus devices and with the time available to bore through a couple dozen miles of rock)

The insueing destabilization of the tectonic activity of the planet completely fubars everything. Of course, the planet is now uninhabitable by anyone SANE, but you could always stick an orbital above it and send down shuttles to dredge for the molten ores now floating on the surface of a planet whose had it's tectonic plates smashed to bits.

This all presupposes a planet with active tectonics, it won't work on one without it. For that you need to drill down to the mantle and drop multiple vortex torps to make the planet implode. Even then, expect there to be survivors. The active tectonics renders survivors moot because all of the tombs will first be crushed to itty, bitty bits, and then flooded with molten rock.

I'm sure that Necrons, and their shenanigans have limits, maybe even similar to the player's shenanigans, but if I think to a couple of the things Necrons CAN do, whole segments of that plan scare me. They might: teleport the bombs away (this could fail, being vortex, but causing LESS damage, so maybe not all bad), materialize gauss pylons to shoot you down long before you bore your hole, or my favorite (as in used it in a story), teleport Necrons aboard the ship. You're sitting there, boring a hole to place your Wile E. Coyote dynamite down, while they are beaming Warriors, Scarabs, and/or Flayed Ones up to your ship, who start hacking through your crew. Depending on your take of their "High Science"/bend the laws of space, time, and physics powers, there might be no way to launch this particular sort of attack, especially now that they think, fear, and plot.

I'm sure that Necrons, and their shenanigans have limits, maybe even similar to the player's shenanigans, but if I think to a couple of the things Necrons CAN do, whole segments of that plan scare me. They might: teleport the bombs away (this could fail, being vortex, but causing LESS damage, so maybe not all bad), materialize gauss pylons to shoot you down long before you bore your hole, or my favorite (as in used it in a story), teleport Necrons aboard the ship. You're sitting there, boring a hole to place your Wile E. Coyote dynamite down, while they are beaming Warriors, Scarabs, and/or Flayed Ones up to your ship, who start hacking through your crew. Depending on your take of their "High Science"/bend the laws of space, time, and physics powers, there might be no way to launch this particular sort of attack, especially now that they think, fear, and plot.shot around the planet. So it's going to hit, and it's void shields and internal armor protecting the device. This smacks into

Slap a bunch of devices that hide it's presence, it's so small gravity isn't going to matter, and then you just position it in some orbit where it's not likely to be noticed, and then activate it remotely at the right time. Hell, it'll take a few MINUTES to drill through crust if you choose a location that's under an ocean. Or you can fire a lance through a sleeping super volcanoe, causing it to errupt, opening up the path for the vortex torp to go in as soon as it errupts.

If you plan in contingencies for countermeasures, sure, as a GM, I'd have that pay off. It probably would anyway.

BUT!

If you don't, though, if I were GMing this, I'd let you run into plausible necron counters to your idea, as far as they are applicable.

So yes, with a proper plan, exterminatus is an option. If your plan only consists of "exterminatus", though, it's unlikely to work without some serious hitches

Primary defence of Necron tombworld: Your ship is met in space by a Necron Scythe class cruiser! Better be on top of your game!

Also, I have my doubts as to whether your Vortex torps would be undetectable to Necron sensors! Their technology is generally far better than yours!

Yeah but see, HOW do they detect it? Necrons work on physical principles, they don't have something like the Eldar who could reasonably 'forsee' the torp coming and counter act it.

Necrons have to detect it. So how do they do so? A torp is too small to put out enough of a gravity interference to be detected from a planet's surface. If you equip it properly then it'd not put out any EM scatter to be noticed either. You can further hide it amongst space debris (which there is always tons of around a planet, even earth has a bunch of small asteroid clouds hovering around it), then you further **** up their ability to detect it.

Throw in a big flashy light show elsewhere in orbit to cloud out their sensors? Sure, they MIGHT detect it, but their chances of doing so are slim to none.

Necrons MUST work on physical principles, it's part of their shtick, even if they have hyper-science, they are bound by HARD science in most cases. They can't even break the light speed barrier on their own, they have to have a C'Tan do it for them.

Throw an active, unbound C'Tan into the mix and all bets are off though.

necrons have not been contacted by the imperium yet? how is this possible yet we still know about them? is RT not at current 40k date yet?

also simple equipment to find would be EMP anything. that oughta stop any necron quickly enough, at the very least stall them before they reactivate, giving precious moments to finish them off

dark heresy and it's associated RPGs are set around 200-300 years before the current date of the table top game.

Yeah, we lose the Imperial heroes, sans Macharius's legend, but it's pretty much the same, otherwise. That way, you can have some "little" warp boo boos, and not end up in the 42nd millennium, saying "oh crap, did Abaddon win?" or whichever angle of 40k you prefer to follow.

It's a shame the Necrons got totally updated, rather than just "added to", as their systems came back online, or these folks could more meet mindless drones, and NOT talk to them. Still, you could do that, and this means that the necrons here predate their bromance with the Space Pretty-Boy Vampires. As an aside, any of their own failings aside, so long as Marneus Calgar never playfully fist-bumps a Necron, that then lives through it, that's one little joy I'll keep in the Calgar vs Dante fanboy war, of which Dante Cullen usually wins, otherwise. Oh well, haters gonna hate, right?

I got a lot of sympathy for Marneus after watching the latest Emp's Text Speech Device episode, admittedly. It's a take on the character I could actually see happening not just to him, but to a fair few space marines who've won a bit too much, or too easily.