Unholy Death Knight

All the covenants are fairly close in value, Necrolord is currently the strongest one, however venthyr gains a lot of value for short AOE burst since the venthyr ability lines up with your other 1min cds.  

Soulbind

The best Soulbind for Unholy Death knight is currently Bonesmith Heirmir Since the crit buffs lines up well with Apocalypse Unholy Blight and  Dark Transformation.

This is what your soulbind tree is gonna look like with everything unlocked

https://www.wowhead.com/soulbind-calc/embed/necrolord/bonesmith-heirmir/death-knight/ApZqAQUoqQYRBSXlBiIVKDQGJSdkBg

Conduits

Potency

For potency conduits you want to go with Eternal Hunger.

Eternal Hunger increases Dark Transformation duration by 3 seconds and your minion damage is increased by 3% as well.

Endurance

Your endurance conduit is going to be Reinforced Shell. It increases the radius by 15% and the duration by 2 seconds of your Anti-Magic Zone ability; this is going to be the go-to choice for raiding.

However, for Mythic+, you may find more success with the Hardened Bones conduit that makes it so while Lichborne is active the damage you take is reduced by 6%.

Finesse

Finesse conduits are a lot more open and really depend on what you need. If there are fights that require interrupts, then the Spirit Drain conduit (successfully interrupting an enemy with Mind Freeze grants 10 runic power) could end up being a very minor DPS increase.

The other option for finesse conduits is Fleeting wind giving you some extra movement speed the first 3 seconds after you press deaths’ advance.

For single target, deadliest coil is going to be the legendary you want to craft. Deadliest coil reduces the runic power cost of death coil by 10 and death coil increases the duration of dark transformation by 1 second. This legendary can be crafted for either the chest or the cloak slot.

For AoE, Frenzied monstrosity is going to be the strongest choice since you will spend your runic power on epidemic instead of death coil. Frenzied monstrosity causes your dark transformation to also increase the attack speed and damage of you and your monstrosity by 12%. It can be crafted for the head or hands slot.

Your bis trinket is gonna be Stone legion healdry from Stone legion generals assuming you have 4 other people in your group running If not then power crystal from De other side is also a viable option.

For your second trinket slot you wanna get Inscrutable Quantum Device which is also from De other side