Ok, where to start.
First, its a neck slot item, which (IIRC) is one of only two slots (head slot being the other, and the head slot item has a boon of the garou clicky, and ferocity IX on it , somewhat wasted on a caster) that a necro can get dark malady in as a focus upgrade in t7.
The neck slot is also one of two slots wherea necro can get a chromatic arc focus upgrade in t7, the other being a 5% drop rate rare ring.
Those things alone makes this the least viable choice for the neck slot, in spite of it being the class specific drop. The neck slot is just plain crowded because of all of the above.
Theres more though:
The focus effect on the neck is blood of the spire.
It reduces spell mana cost of dread pyre by 25 %.
Dread pyre is an 18 second base dot, that I can't see any necro even having on the spellbar just entering into t6, let alone t7. There are several better spells that could and do take its place, justifiably, on the spellbar.
My humble suggestion, based on whats available for focus items in t7 that a necro could and would use:
First, make it wrist slot, or ear slot.
Second, change the focus on it to something other than what it is.
Looking at t7, you have pet focus in the face slot. The only option.
You have other focus options for fire, magic, distance, swiftness, malady, bile, savings (beneficial), preservation (detrimental), etc in the finger, neck, shoulder, ranged and head slots already.
On the other hand, there is little to no access to dark health or dark quickening , for a necro in t7. A rare high ac bracer off koritz with dark health, or a rare high ac high parry mod clicky slow earing with dark quickening off another t7 boss, visath iirc, or the rare arms off adrianna, which have the DPS clicky on them.
Wizards got dark fire with a clicky nuke on their class earing, by comparison.
A wrist or ear slot, with dark health or dark quickening (Either would be spectacular) and perhaps an ancient curse of mori (or whatever dot would be appropriate) click effect would be a much better parallel to the wizard class ear for example, and would fit both focus wise and slot wise, with whats available in that tier, without being overpowered.
As it sits currently, its just not a viable item, in the neck slot, or with that focus. It just doesnt fit with the other focus upgrades in the tier either way.
Regards,