Hello! Riskee here, a devout arcane mage with some feedback about necrolord!
I’m going to start by saying, I love necrolord and would swap in a heartbeat if it wasn’t so far behind in DPS, and I hope to address what I think are the primary issues holding back this covenant choice for mages. Do note however that I play arcane, so players of other specs feel free to add to the discussion! 
Let’s cut to the chase: Deathborne is weak. It’s unfortunate, but it’s true. Compared to the other covenant abilities Deathborne is pretty terrible. What’s exactly the problem here? Well there are two problems I see with it.
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The cooldown. 3 minutes is a long time. For Arcane this means the ability will only sync with every second arcane power. As an ability with low impact (compared to the other covenants), this is pretty bad. If the ability’s cooldown was moved down to 1.5 minutes it would align with every single arcane power, and thus have more impact on our gameplay. (2 minutes would also cause it to be held for every other arcane power since with the Arcane Prodigy conduit, our Arcane Power is reduced to about a 1.5m CD).
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The buff it grants is too weak. Compared to the other covenant abilities (Mirrors of Torment, Radiant Spark, and Shifting Power), Deathborne has no real impact on our gameplay and isn’t very strong. In most raid situations it’s just a straight up Spell Power buff with a cast time. This is tragic because it is one of the best abilities visually speaking added in the entire expansion. Realistically, I doubt the ability will get a rework so an increase in the buff it gives would be nice, or even adding in secondary stats or something. Anything really!
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Gift of the Lich is awful. Gift of the Lich is the Necrolord Conduit and what does it do? Increase the duration of our Deathborne. That’s it. For a weak ability that doesn’t really play into the strength of the class, increasing duration is marginally useful for a class that relies on stacking all of our CDs for a huge and short burn phase. I would love to see this conduit reworked to increase the buffs Deathborne gives rather than duration. Because as it stands, keeping Deathborne active for a bit after arcane power is really not useful. This conduit was so bad they buffed it by 100% and it stil isn’t even good. Maybe if the conduit extended duration by a set amount and then have a spell power buff that scales with ilvl of the conduit would be a good way to fix this.
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The cleave Deathborne grants us isn’t even tied to our primary spell. The past two expansions our damage has primarily come from arcane missiles, not arcane blast. The fact that Deathborne’s cleave is put on Arcane Blast only is pretty disappointing. I would love to see the cleave to be added to Arcane Missiles as well for arcane (I can’t talk for fire or frost, but I assume adding the cleave to flurry/pyroblast would be beneficial for them anyways). If the goal was to let out primary ST spells cleave then the ball was completely dropped here. Our talent row that has our cleave/aoe talents (Arcane Echo and Resonance) isn’t even utilized by Deathborne, since you’re either focusing on barrages for reso or spamming missiles for echo.
And there it is, my probably way too in-depth issues with Deathborne.