A core choice at the start of your journey in the Shadowlands will have you decide where your allegiance lies; would you rather support the Kyrian of Bastion, the Necrolords of Maldraxxus, the Night Fae of Ardenweald, or the Venthyr of Revendreth?
Choosing a covenant is the most important decision your character will make in Shadowlands, as the covenant abilities and Soulbinds will significantly impact your spec's gameplay and performance. Through this choice, your rotation changes, certain bonuses appear in dungeons for your entire group, and you are able to follow the story in different paths.
So without any further ado let's get right into it;
Currently all four covenants are actually a viable option for Blood Death Knights with Kyrian being the most popular option.
Necrolord is also a very strong option on boss fights that require add management such as Soulrender and Fatescribe
AoE / Mythic +:
Again all four covenant choices are very good for Blood Death Knight but when it comes to Mythic + Venthyr is the most popular choice.
For PvP you want to Necrolord as the covenant signature ability Abomination Limb is a very strong crowd control ability.
To sum this up, all four covenant choices are very viable for Blood Death Knight and should be chosen based on the current preferred content of choice or aesthetics
The most popular covenant for Frost Death Knight is Necrolord as it is very versatile and Abomination Limb can really shine on a couple of fights, Soulrender Dormazain for example and it it the most popular Frost Death Knight covenant choice.
AoE / Mythic +:
For Mythic + Necrolord Frost Death Knight really shine due to Abomination Limb allowing you to grip multiple targets for massive team AoE damage and control.
Venthyr is a close second when it comes to Mythic + content as the Runic Power provided from Swarming Mist allows you to keep your Breath of Sindragosa running for a longer period.
For PvP your only viable option is Necrolord, Abomination Limb really compliments Frost Death Knight PvP playstyle and it is your bread and butter when setting up kills. As well as Felshcraft giving you more tankiness allowing you to stay in the fight more.
For every aspect of the game Necrolord is your best option as it is just versatile and good in every single scenario.
All four covenant choices are viable on an Unholy Death Knight with the exception of Necrolord being the more popular / versatile option and should be taken over the other if you would like to min max your character.
AoE / Mythic +:
And for Mythic + / AoE Necrolords is still your best option due to how strong Abomination Limp is and being able to grip in multiple targets making it the best option for Mythic+ and AoE scenarios.
Venthyr is a second good option since Swarming Mist better lines up with your shorter cooldowns.
For PvP Necrolord offers the best offensive and defensive choice for Unholy Death Knight and Abomination Limb being a really strong crowd control ability making Necrolord the go to covenant for PvP.
To quickly sum this up, for most aspect of the game for Unholy Death Knight Necrolord is your go to covenant as its very versatile and compliments the Death Knight playstyle.
Covenants have a major impact on the game, your playstyle, rotation and even character aesthetics. You should consider all your options before committing to one covenant.
But also remember you can switch between covenants so if you would like to change between them you can!
As of current there is a time gate on how you can switch covenants but with the release of patch 9.1.5 it will be much easier
Remember! You will lose access to all your previous Covenant’s abilities, transmogs, and soulbinds
It is a bit tricker to rejoin a covenant you have already left and it will require a bit more work. In this case, you will need to complete two consecutive weekly quests before you will be allowed to rejoin.
The leader will then give you a weekly quest to start the process of rejoining the Covenant by completing quests in the covenant zone or dungeons and you will see the progress bar fill up as you finish various tasks.
Once you finished the quest and the Weekly Reset has happened you will be given yet another quest, the objective of the quest is pretty much the same then you will be able to rejoin the covenant.
In total it takes about one weekly reset to rejoin a covenant you previously left, so make wise choices!