A word of warning to this page, if you don't like complicated as fuck models, you will rage. But at least this one is less croissant-shaped than other Necron models.
These spiders of doom are Controlled by a high-ranking Triarch Praetorian. They are equipped with Quantum Shielding and armed with slicing limbs and devastating weaponry. Triarch Stalkers are extremely maneuverable and quick for vehicles of their size, capable of covering all kinds of terrain.
Triarch Stalkers are giant Necron metal spiders. They are traditionally used to perform flanking and envelopment maneuvers, but can do a surprising amount of damage when deployed on the front lines. They can wield a fusion laser cannon that can either be focused into a medium range, melta-style cutting laser, or a wide, fanned out blast of boiling plasma. Their advanced targeting relays can bolster any nearby Necron units' accuracy programming.
Tldr: It's a walking tank with capability similar to a Predator Annihilator, with a few extra Necron tricks thrown in.
The Triarch Stalker is a walker that ignores Crew Stunned/Shaken, has Move Through Cover, and possesses Quantum Shielding, giving it AV 13/13/11 until the first penetrating hit, all for 125 points. It carries the only Necron melta weapon called appropriately enough, the Heat Ray, which it can alt-fire as a heavy flamer (making it one of the the only Necron weapons able to pierce through the bullshit 3+/2+ cover every point-scoring unit have nowadays), and grants all allied units around it +1BS, for that extra dollop of "fuck you." It makes 3 attacks at S7 in close combat, and only the hidden power-fist (or equivalent) can really scare it. You can swap out its Heavy 2 melta laser for a twin-linked Heavy Gauss Cannon (S9 AP2 36" Assault 1) or the Particle Shredder (See: heavy psycannon) (S7 AP4 24" Large Blast). They can deploy in a squad of one to three, and form the linchpin of an excellent Decurion formation.
In 8th edition it was a solid unit for the points. Not all of its weapons were as useful as others, but it was a very useful support unit due to its Targeting Relay special rule already. This rule gives reroll 1s to hit for anything with the NECRON keyword that is targeting a unit that has previously been shot at by a Triarch Stalker during the current shooting phase. It therefore pairs well with a Doomsday Ark for decent anti-vehicle goodness.
9th Edition Zoomer Primer
Everyone's best friend is probably better than ever in 9th Ed. It still provides the same reroll bonus and is now one of the rare no-conditions buff in the Codex/game, giving rerolls of 1s to hit to any NECRON unit that shoots at what you just managed to hit. While it's other main rule, Quantum Shielding, has received some change, they haven't significantly affected the mathhammer of it.
As for the Stalker itself it has received nothing but buffs, the most straightforward being an extra 2 wounds, bringing it up to 12. Every single one of its guns has also been buffed, making the choice of it extremely tough.
- Heat Ray's range has been buffed - it is now 2D6 shots of S5 AP-1 2d at 12" _or_ 2 shots of S8 AP-4 D6d at 24". Flexibility is always good, and this one fits especially an aggressive playstyle against hordes based armies. A somewhat situational other advantage is that the sweeping close range profile autohits like a flamer, meaning that it doesn't suffer if you operate at a lower bracket BS.
- Particle Shredder lost a pip of strength (S6) but gained two shots (8!) and hits with flat 2 damage at 24". This makes it almost perfectly designed to knock out 2W marines and elite infantry in general.
- Twin Gauss Heavy Blaster is now 6 S7 D3 damage with a beefy AP-3 at 30". While its damage is less dependable than the Particle Shredder, it hits targets harder and at longer range. This also gives it the edge if your goal is merely to soften up a target and apply the reroll rule so your Doomsday Ark or Doomstalker can obliterate it more easily.
Its melee weapon is really beefy as well, hitting at S7 AP-3 3d. With three such attacks, it can seriously bust through vehicles or monsters, though its number of attacks means it can still get tarpitted. At least it can no longer get tripointed and has stratagems to get out of dodge.