Preface:
Hello, I would like to propose some ideas for reworking the Nightblade class. Currently, this class is positioned as a typical stealthy assassin, and its skills have a solid concept, but some skills lack variety and fail to fully realize the class's potential. Its skills are mostly related to shadows, stealth, and draining enemy power, but this could be deepened by adding a few more features — dark magic (Mor), poison, and bleed. For dark magic, a distinct dark effect is needed, aimed at magic-based builds, to unlock the class's magical potential. Poison and bleed are obviously meant to unlock the assassin's potential. I have tried to create a few visual interpretations to make it easier for you to understand what I mean.
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New passive abilities:
New Mor Effect (Dark Magic):
Some Nightblade abilities now apply the Mor effect. This effect has no positive or negative effects on its own, but it is required for the activation of certain other abilities. An enemy can have a maximum of 6 Mor effects on them at once.
New Bleed Effect:
Bleed deals periodic damage. The Nightblade can apply Critical Bleed (depends on critical strike chance), which deals more damage based on the critical damage stat.
New Poison Effect:
Some abilities can apply Poison. The poison effect increases the damage taken by the enemy and ignores N amount of the enemy's armor.
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The skill Veiled Strike is mostly used in PvP and combined with the invisibility skill (Shadow Cloak). Its feature is simply damage and a bit of crowd control in one of its morphs, but outside of PvP it feels too boring and not particularly stylish. Therefore, I suggest slightly reworking its feature and visual effect.
New feature:
The skill now deals damage with two blades and applies bleed. When used while invisible (Shadow Cloak), it guarantees a critical hit.

First morph (Stamina):
Now costs Stamina. Increases bleed damage when striking from behind or from the side.

Second morph (Magicka):
Now deals AOE damage, with increased radius. No bleed effect, but instead applies a Mor effect to the enemy(maximum 3). If the enemy has 3 Mor effects, the ability consumes all of them and deals additional damage. If an enemy has 6 Mor effects on them, the ability "detonates" the Mor effects and deals high periodic damage for each effect.

Visual concept of the new skill:


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The ability Path of Darkness is focused on increasing movement speed, and its morphs either deal damage or provide additional buffs. This ability is the most fitting for the visual effect of Dark Magic and has greater potential.

New feature:
Path of Darkness now creates a Dark Mor on the ground that deals damage to enemies within its radius. It applies Mor effects to enemies (maximum 3 stacks).

First morph (healing and buffs):
The skill now revolves around the character. It deals no damage and does not apply Mor stacks to enemies. Allies within the ability's radius gain a blood bond with the character. The blood bond heals allies and increases Magicka and Stamina recovery.

Second morph (Magicka damage):
The ability has a larger radius and now applies 6 stacks of Mor. It deals periodic damage within its radius. This ability can be combined with the Veiled Strike skill in its magicka morph, which requires 6 stacks of Mor for burst damage.

New visual concept of the new skill:


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The skill Aspect of Terror is used for crowd control and debuffing enemies, and it is also mainly used in PvP. However, in my opinion, this skill should be offensive and could have more variety.

New feature:
The skill now has a different visual effect — the character throws several daggers in a cone-shaped area in front of them. The daggers are coated with poison and apply the Poison effect to the enemy.

First morph (Stamina):
The skill costs Stamina and deals periodic damage, while still applying the Poison effect to the enemy. With each tick of periodic damage, the enemy takes increased damage. (The Poison effect ignores N amount of the enemy's armor.)

Second morph (Magicka):
The daggers are now imbued with Dark Magic, have a larger radius, and feature a different visual effect. The skill applies 3 stacks of Mor on enemy. The ability deals periodic Magic damage that scales with the number of Mor stacks on the enemy (maximum 6).

New visual concept of the new skill:


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The ability Teleport Strike simply deals damage and has no additional effects, and it is also mostly used in PvP. I think its PvP potential should be preserved in the Stamina morph, while the Magicka morph could have potential in PvE.

New feature:
Teleport Strike now applies the Poison effect to the enemy. If the enemy dies, the poison effect spreads to nearby enemies around them.

First morph (Stamina):
When activated while invisible, it knocks the enemy off balance and applies the Poison effect. If the enemy already has the Poison effect on them, it deals periodic damage based on the armor penetration rating.

Second morph (Magicka):
Now, when activated, a Shade version of yourself attacks the enemy. The ability becomes ranged. After being used, the Shadow leaves a Dark Mor on the ground and deals periodic damage in a circular area. If the enemy has 3 or more Mor stacks on them, they receive an armor penetration effect.

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The ability Cripple simply deals damage and, in my opinion, doesn't look very stylish. This ability could also be connected to the character's Shadow and be more interesting.

New feature:
Your shadow appears behind the enemy, deals damage, and applies Bleed. It deals periodic bleed damage (can apply Critical Bleed, which scales with the critical damage stat).

First morph (Magicka cost):
Your shadow's blades paralyze the enemy.

Second morph (Magicka):
The ability can consume Mor stacks on the enemy. Upon activation, it consumes 3 Mor stacks and increases the periodic bleed damage. If the target dies, the bleed effect spreads to nearby enemies. This ability cannot apply Critical Bleed.

New visual concept of the new skill:

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New feature:
Deals direct damage. If the enemy has a Bleed effect on them, the bleed damage becomes instant and heals your character.

First morph (healing):
The ability can now heal allies and apply Blood Bond to them (similar to the healing morph of Path of Darkness). If an ally already has Blood Bond on them, they instead receive more healing and cleanse all negative effects from themselves.

Second morph (damage):
The character turns to dark magic and applies Mor stacks to the enemy. If the enemy is under a Bleed effect, the bleed damage becomes instant and deals additional direct damage for each Mor stack on the enemy (maximum 6).
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This is an approximate visualization of the visual effects. I will add to this post as I have time, but I think the main idea I want to convey is already clear. Dark Magic and Shadow-based abilities make the Nightblade's mechanics more interesting, unlock its role as a stealthy assassin, and allow the potential of a Dark Mage to be realized—something that is sorely lacking in ESO. You can combine Mor, Bleed, and Poison effects in different ways to create gameplay that suits your own style.

Conclusion:
I hope my ideas help the developers find a unique formula for reworking the Nightblade's skills. I like the new direction the developers have chosen, and I sincerely hope their efforts pay off. It's rare these days to find passionate developers who are genuinely invested in their project.

If you have your own ideas, feel free to share them in the comments.