Minmaxed necro tank with the most supportive abilities from necro class, snipped redundant skills from most popular builds, and streamlined cp allocation to more accurately reflect skill choices.

No Pierce Armor as Unnerving Boneyard serves the debuff function on an aoe and Inner Rage replaces the taunt at range. All around better combo, though the alternate Boneyard morph is feasible with Roar of Alkosh gear for raw DPS power. That takes a while to get, so I dumped it in favor of freeing a skill slot. Be advised that if you dump the Unnerving morph, you *will* be less efficient in tanking, as you'll have to drop a skill to pick up Pierce Armor.

Heroic Strike is the only non-class source of maim and it generates ultimate, so dumping the S&B is absolutely not an option. Plus, the passives towards blocking will come in handy when you're about to have your butt handed to you on a silver platter.

Elemental Blockade can set off balance (with the passives) and proc weapon enchants, which means it's good to throw an oblivion damage enchant on the destro staff for free DPS. Might as well, right?

If Mystic Siphon isn't working out for you, consider picking up Deep Thoughts or Spell Symmetry. I've found that (granted, this isn't always the case with PUGs) when the healer/DPS has Necrotic Orb or Siphon Spirit, I rarely ever have to heavy attack with the destro staff. This... is the best-case scenario.

Spirit Guardian w/ undead confederate for resource regen and damage mitigation, and Necrotic Potency for an in-combat HoT and more ultimate generation.

Agony Totem and Aggressive Horn will aid in DPS while simultaneously providing a supportive function to your party in the form of damage taken reduction, crowd control, resource generation, overall DPS increase, or some combination thereof.

Beckoning Armor is your main "oh hecc" button (also useful for pulling ranged stragglers that you just so happened to taunt at range with Inner Rage--funny how this all works together ain't it?), and Ravenous Goliath for when stuff really starts gettin' wild.

A Necro tank is more of a support tank than anything. Enough self-sustain to lessen the healer's job, and enough group mitigation to further nail that point home. Solid aoe damage and decent (de)buffing.
Heavy Ebon Armory and Mediumx1/Lightx1 Lord Warden gear is absolutely the way to go.

This build can survive ALONE in vet dungeons for as long as your Ravenous Goliath is up. You can absolutely save a run from wiping on a difficult boss if you keep a level head. If mana sustain proves to consistently be a non-issue, I'm going to swap over to 2H axe for a mondo single-target DPS increase. If you make the swap, you're gonna want to balance your stamina use with light/heavy weaving with Executioner after 50% hp down.