The more Dakka version of the Canoptek Reanimator. The
Martian Tripod Canoptek Doomstalker is basically just a scaled up reanimator with a big fuck off gun duct taped on top. As you can imagine, its another one of those units shamelessly ripped off from HG Wells War of the Worlds, as if GW needs any more of Wells' ideas. We are literally just a few units off from becoming a literal War of the Worlds tabletop, as if it already doesn't exist.
Canoptek Doomstalkers are giant Necron machines, armed with a back-mounted Doomsday Blaster that can be fired on the move, or they can remain stationary to unleash its full cataclysmic potential. Should any foe get too close to a Doomstalker, the machines can unleash their auxiliary twin Gauss Flayers to defend themselves.
In terms of their roles, these War of the Worlds prop act as the guards and defences to the Tomb Worlds' armouries and the Canoptek Plasmacytes - which was originally created to siphon off corrupted engrams from sleeping Necrons in stasis crypts though they have been repurposed by the Destroyer Cult to now do the reverse.
While initially looking like a discount Doomsday Ark - quite literally, since it's 50 pts cheaper - the Doomstalker actually has enough to set it apart to make your choice of Doom Dealing Heavy Support quite competitive.
First of all, the main gun. It's the exact same as the Ark, with a bit less range.
What's quite different is literally everything else. First, you're not a vehicle, you're a horrible monster. But at least a Canoptek one, meaning your list of synergy is completely different - most importantly you love Technomancers. You do degrade, but in a very different and in many ways superior way to the Ark, since your slightly worse BS doesn't falter (thank god).
Your movement is actually really good (10"/8"/6" for each bracket), you do not have Quantum Shielding, but you do have a 4+ Invulnerable Save per your Containment Field rule, on top of a really decent base save. You do not however have fly, so no hopping over terrain features. Very sadly, you do not have a melee weapon either, so your couple of S6 attacks will not pack much of a punch at all nor do you have the sheer number of Gauss Flayer shots the ark carries with it - meaning you really do entirely depend on your ability to get the Blaster on target.
I wasn't kidding earlier, you really love Technomancers. So much so that you have a stratagem that allows you to fire back in revenge whenever a friendly character is killed, as well as gaining +1 to your hit rolls against the killer. True love. But probably your most powerful rule is that you can fire Overwatch on behalf of any unit within 6" of you. This encourages you even more to move your ass up, stand behind a screen and blast every vehicle you can see into dust with your mighty Doom Chode.
So in the end, the Doomstalker is a much more aggressive, much more itching to fire version of high-power heavy support that if properly protected will pull immense weight.