Usually when someone says Nightblade, they mean the Nightblade line. Silent Blade and Acheron will both do more damage than a 3* Nightblade.

Obsidian Edge would require the Apocrea to come back, so unless you already have the materials, I think you're stuck with the Acheron. I'd definitely pick the Acheron over the Gran Faust though. Obsidian Edge I guess would be safer, but either 5* swords will do just fine.

If you're going for sheer speed tho, the fastest runs I've seen use the Fang of Vog. There are a few demonstrations on youtube like this and this.

You usually see Nightblades and Silent Nightblades just because people haven't got the resources to fully upgrade it yet.

Another reason you don't see many (new) Acherons is because of the difficulty obtaining radiants. A level 10 Silent Nightblade does as much damage as a level 7 Acheron, and it charges faster.

The Obsidian edge is a much more balanced and defensive weapon than the Acheron. The poison can cut back enemy attack power while reducing their defense. It can also reverse or prevent healing.

@ Menish

I'm leaning towards the Obsidian edge, whatever damage you deal with the Acheron can be quickly healed back by the minis. If you prevent them from healing the JK then you get full damage

(Acheron damage - mini heal rate) VS (Obsidian edge damage)

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Of course there is also the possibility the player has a Maskeraith or poison vials.

In general, Acheron and Obsidian Edge are fairly balanced. Usually Acheron is more useful, because it inflicts so much damage. But sometimes Obsidian's poison can be really useful, especially in a party where several knights benefit from it.

For the Royal Jelly, both of them are overkill. You should be able to destroy RJ rapidly with either one. Or Gran Faust. Or Warmaster Rocket Hammer. Or Combuster. Or certainly Fang of Vog charges.

If you're looking for other RJ tips, then definitely try to load up on pills and pickups for poison, fire, shock, and (if you can get it) curse. Using these you can kill RJ pretty easily with just about any non-piercing sword.

Straight up damage the Acheron wins, but if you have a JK constantly getting healed faster than you can damage it, or a Giant lichen with 5-6 silkwings healing like crazy then your Acheron isn't worth as much.

The Obsidian Edge may not have as much oomph but when there are healers around power isn't worth as much, you will want that poison to reverse the healing and to dampen incoming damage.

Still at the end of the day the best weapon for JK is the Dark Retribution.