With subclassing, everybody is taking a closer look at every classes skill lines, and how...HOW has Dark Magic remained like this since launch basically? Not just the passives, but the skills too!

Encase: Call forth Daedric shards from the earth to immobilize enemies in front of you for 4 seconds. Enemies hit are afflicted with Major Maim, reducing their damage done by 10% for 10 seconds.

The morphs are hardly any better...one that does poo damage after 4 seconds (this isn't a delayed burst like Haunting Curse or Purifying Light), and the recent addition of Vibrant Shroud, which may have SOME applications, but is prohibitively expensive. And then it kind of has an identity crisis on if you want aim it at allies for the heal/buff or enemies for the debuff... And it's still outclassed by Combat Prayer in most (all) scenarios.

Rune Prison: Imprison an enemy in a constricting sphere of dark magic. After a short duration they are stunned for 3 seconds. This stun cannot be blocked.

This is probably even worse. A delayed stun that does no damage by default, and nothing else. One morph adds (again) a negligible amount of damage if the enemy is stunned for the FULL DURATION (in PVP, where this is realistically the only application for this skill to begin with, nobody is just going to eat the 3 second stun). And if the damage does actually trigger, it's nothing to even be remotely concerned about. The other morph auto stuns the next thing to attack you...which is pretty much worthless.

Daedric Mines: Surprise your foes by placing 3 volatile Daedric mines around you, which take 3 seconds to arm and last for 15 seconds. When a mine is triggered it explodes, dealing 2613 Magic Damage and immobilizing the enemy for 2 seconds. Enemies can only be damaged by your mines once every 2 seconds.

This is an awkward, strange skill no matter how you look at it. Expensive as all heck, lackluster damage (with a 2 second cooldown), and contingent on NPC's stepping on your mine, or players. But also, they have 3 seconds to NOT do so, so there's really nothing "surprising" about this skill at all. Daedric Refuge, the new damage shield morph, was an interesting idea, but still suffers the awkward application and is virtually never used.

Passives:

Unholy Knowledge: Reduces the Health, Magicka, and Stamina costs of your non Core Combat abilities by 6%.

This is fine, pretty similar to other classes'.

Blood Magic: When you cast a Dark Magic ability with a cost, you heal for 1600. This effect scales off your Max Health.

This could be okay, but I think it needs a higher base heal, or scaling with your highest stat in general (Health, Stamina, Magicka). OR, consider removing the Dark Magic requirement (better for Subclassing) with a lower heal. Something akin to Reaving Blows CP (not damage %), but weaker.

Persistence: After blocking an attack, your next Health, Magicka, or Stamina ability costs 15% less.

Why? It's rather niche and more favorable for tanks, and there's already a cost reduction passive in the same skill line.

Exploitation is a run of the mill Minor buff to offer the group that most classes have in some form. No comment.

Most classes have rather specific intentions for each skill line (a damage line, healing line, tanking line). There's some overlap, sure, but they're more tailored for their intent. Dark Magic has zero real identity...it's a mess. And honestly, Daedric Summoning suffers from a similar identity crisis.