It is one of the, if not the worst covenant abilities in Shadowlands. I would actually argue that it is the worst. It has next to zero functionality on anything other than stationary boss fights. Even then, you have to stop attacking the boss to use it.
Everything about it feels bad to use:
It does not activate the buff on use. It activates After you Kill the Demon it spawns. So you cant count on it for Defensive usage. There is a delay to it, that is not consistent like sigil activation.
It is a stationary buff for one of the most mobile classes. You have to stand in a puddle for the buff. The buff is 15 seconds, so if you use it at the start of a trash pack, you have a chance of using it fully. But any movement can negate the buff and waste the CD. It also gets lost in ground clutter. Sanguine, Quaking, Volcanic and all other spell effects can get lost in it, which could kill you. Need to move a boss for a mechanic? You lose the buff.
It only affects the caster. If it buffed the whole group, it may have some merit, but its just a 15% dmg reduction for the caster.
3min CD for a 15 Sec Buff. Unless WoWhead has it wrong, a 3min CD for a 15sec buff, that you most likely will not see the full duration of?
Due to how bad it is, Necrolord is a dead covenant for Demon Hunters.