The Dark Knight Zerg
This guide may contain some out of era so please double check the info provided in this guide.
What is a "Dark Knight zerg"?
Simply put, it is combining the abilities Souleater and Blood Weapon; this allows even weaker weapons (eg, Kraken Club) to be really mighty, whereas your Blood Weapon prevents your HP from dropping (and hence your Souleater power does not decrease)!
There are two main ways of zerging; one-handed and two-handed. The general subjob used for two-handed zerging is /sam (due to Hasso) and the one for one-handed either /drg (due to Wyvern Earring) or /nin (survivability).
The Basics: Stat Priorities/Weapons
Two-handed weapons: Rune Chopper, Fortitude Axe, Bahamut's Zaghnal
One-handed weapons: Ridill, Mercurial Kris, Kraken Club
Four of these five weapons have one thing in common; "Occasionally attacks 2-x times". This is very important since during a zerg, it is the QUANTITY of the hits which matters ultimately. When you are attacking at maximum speed, it is those "occasionally" hits which really push you over the top.
Souleater also contributes the majority of the damage/hit to these weapons. Therefore, amplifying Souleater damage by adding HP does far more to your damage than amplifying the "base damage" by adding atk/str. So,
To summarise: Haste > HP > else for a zerg.
The current haste cap is 80%. Equipment haste caps at ~25.25% (sources vary, but since you need 26% to cap it and all sources agree on this, it is irrelevant). Magic haste caps at 43.3%. Hasso adds an additional 10% haste for two-handers, and Desperate Blows adds an extra 15% for two-handers external to either magic or equip haste. Therefore, we build to maximise haste and HP, with haste as the priority. Two-handers have more flexibility concerning gear options due to that 25% haste from Hasso/Desperate Blows!
Additionally, it is very good to consider adding a piece of equipment which amplifies Souleater effect (from 10% to 12%) -- either Chaos Burgeonet or its HQ, or Gloom Breastplate, or Sable Cuisses. Gloom Breastplate is best in a low-haste setup since you cannot achieve any haste on the body, but in more flexible setups Sable Cuisses or Chaos Burgeonet can be more beneficial.
Food: Carbonara is attractive because it gives lots of HP and to a lesser extent, attack.
Main: Weapon of your choice.
Sub: Pole Strap, the 2% DA will outdo even the HP on the Axe Grip or other alternatives. If you are one-handed zerging, Januwiyah (a shield) or its HQ for the HP.
Ranged/Ammo: Fenrir's Stone if it currently gives you HP or Happy Egg, though virtue stones if you're using a Fortitude Axe obviously.
Head: Walahra Turban/Chaos Burgeonet or its HQ. Walahra Turban has both 30HP and 5% Haste, Chaos Burgeonet has your Souleater bonus.
Neck: Ajase Bead Necklace or Shield Pendant have HP, as does Ritter Gorget. Peacock Charm if you need accuracy (less supported zergs perhaps).
Earrings: Cassie Earring or Intruder Earring, then Brutal Earring OR Wyvern Earring if /drg (otherwise Cassie/Intruder or other HP earrings). Do NOT use a Brutal Earring with the onehanders or Bahamut's Zaghnal; when you are attacking at such a high speed the higher end of the "occasionally attacks 2-x times" will proc often, and forcing a Double Attack before this is counterproductive.
Body: Gloom Breastplate for the Souleater bonus, or Homam Corazza to add some Triple Attack to the build (only do so if you can put Souleater bonus on either the head or the legs and still have capped haste!)
Hands: Dusk Gloves or their HQ. HP and Haste.
Rings: Bomb Queen Ring and Bloodbead Ring. An Alternative to the very difficult to get Bloodbead Ring is a humble Bomb Ring. If you are not capping haste, Blitz Ring is a very good choice; the 1% haste will help you more than any other ring if you are not capped.
Back: Gigant Mantle or High Breath Mantle. Lots of HP.
Waist: Swift Belt or Speed Belt or Velocious Belt. Lots of haste from this slot, and not much else you can do with it.
Legs: Homam Cosciales for haste, Dusk Trousers for HP, Sable Cuisses for Souleater effect.
Feet: Homam Gambieras for HP/Haste/Acc. Dusk Ledelsens are an alternative. Marine M Boots or Marine F Boots are very viable if you have capped haste already.
If we are two-handed zerging with good support, we assume that we have 10% haste from Hasso, 5-25% haste from Desperate Blows and 43.3% haste from magic. That means that we need 22-2% of equipment haste to cap it to 80. (80 - 43.3% - (5-25%)). If you are onehanded zerging, you obviously need more haste than you can get from equip (37%), so you have less luxury to swap around equipment.
A note on the Rune Chopper: this weapon since the latest haste nerf is only really good when you can't get enough haste to cap effectiveness (since it gives 9% haste, you only need 3% to cap from equipment!). When you can, it is inferior to many standard weapons, including a relatively weak Great Sword zerg. Consider it the stepping stone to a Fortitude Axe zerg for two-handers.
Order of abilities
Seems like a fairly obvious point, but in what order are the abilities used? Obviously the earlier you engage your enemy, the more damage you do, and choosing your abilities in the right order can save you a second or two; which can mean a lot.
If the enemy is not immune, begin with Dark Seal + Drain II. After that, it comes down to personal preference: I prefer to use Hasso (if /sam) => Blood Weapon => Souleater => Last Resort. The logic behind this is thus; you will be fully buffed before engaging and so will be hitting very fast; if you activate Souleater first, you will hit before activating Blood Weapon and so lose HP, so your first few hits will be weaker. Last Resort is very important if you are zerging 2handed so you want to get the full 30 seconds of benefit on that, and in a zerg situation mages can support your HP loss quite comfortably for the 2 seconds or so of HP loss you get from using this order.
Again, this is just personal preference, but that is my logic behind it.
How do they compare?
For one-handed, Ridill < Mercurial Kris < Kraken Club.
For two-handed, Rune Chopper < Fortitude Axe or Bahamut's Zaghnal although Bahamut's Zaghnal may have a slight advantage.
As for how they compare to onehanded weapons,
Rune Chopper < Ridill < Fortitude Axe or Bahamut's Zaghnal < Mercurial Kris < Kraken Club.
Depending on setups and amount of support and even luck with the "occasionally attacks 2-x times", Fortitude Axe and Bahamut's Zaghnal are roughly on par, although Bahamut's Zaghnal may have a slight advantage.