Where the Immortals are the Necrons' Shock troopers who supply heavy fire power, the Deathmarks bring elite, surgical threat removal to the board. Sixty million years ago, when the Necrontyr were still made of meat and blood, the Deathmarks did more or less exactly what they do now: appear from the shadows, kick the fuck out of their target, and then leave. The Deathmarks received high-quality shells during bio-transference, and thus have remained largely unchanged through the millennia. But military honor tradition among the Necrontyr impose harsh restrictions upon the deployment of Deathmark squads. Among each other and any 'honorable' alien opponents, Deathmarks are taboo. But most Overlords see other races as ignoble and inferior, so few balk at the idea of deploying the assassins, except for Nemesor Zahndrekh; the crazy old coot won't use Deathmarks against anyone, because they're all Necrontyr to him.

Deathmarks don't deploy to the field of battle with the rank and file; instead, these cyclopean laser-rapebots spend their time in the inter-dimensional break room watching their intended target on 4-dimensional wide screen monitors and doing fuck all until the commander summons them, or whenever the fuck they feel like doing something. Typically they are left to their own devices, the nobility having great faith in their long practiced art of death. Once it's time to get shit done, they place a hovering green halo of light on the head of their target or targets, thus the name Deathmarks.

On the tabletop, Deathmarks operate best in smaller squads. If you have 10, field 2 squads of five instead of 1 squad of 10. That way you can make better use of your firepower, because the turn these guys Deep Strike they wound EVERYTHING on a 2+ their other special rule is Ethereal Interception, which allows any unit of Deathmarks in reserves to immediately come in via Deep Strike in response to any enemy unit coming in from reserve. They can do this on the enemy turn. Space marines Drop Pod in? Interception. Creed outflank some Baneblades on you? interception. Stormraven-dropped Terminators got you down? Intercept that shit. On top of all that juicy goodness,if they [Deep Strike] during your enemy's turn you can SHOOT DURING THEIR SHOOTING PHASE (but you can't shoot on your next one, to solve the hits just do like in close combat solving all the attacks, and then removing models from play) their 24 inch range sniper rifles are Rapid Fire, a level of shootiness so delicious that 6th ed Tau ripped it off as soon as they saw it. For 18 points a model you better goddamn field some Deathmarks, unless having 3 C'tan Shards is the only thing that does it for you anymore.

They are not an easy thing to live with.

Forces of the Necrons
Command: Cryptek - Destroyer Lord - Necron Lord - Necron Overlord - Phaeron
Troops: C'tan Shards - Deathmarks - Flayed Ones - Immortals
Lychguards - Necron Warriors - Triarch Praetorians
Constructs: Canoptek Spyder - Canoptek Wraith - Crypt Stalker
Scarab - Tomb Sentinel - Tomb Stalker - Seraptek Heavy Construct
Vehicles: Annihilation Barge - Catacomb Command Barge - Doomsday Ark
Ghost Ark - Monolith - Sentry Pylon - Triarch Stalker - Tesseract Ark
Flyers: Canoptek Acanthrite - Doom Scythe - Heavy Destroyers
Necron Destroyers - Night Scythe - Night Shroud - Tomb Blades
Abattoir - Æonic Orb - Doomsday Monolith - Megalith
Necron Pylon - Obelisk - Tesseract Vault