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There's quite a few interacting rules here, so there will be some quotations. It's all necessary to see the interactions.

I have a Pure Nurgle Daemon Detachment. The Locus of Virulence is in effect, which states, "Each time you make a wound roll of 6+ for a NURGLE DAEMON unit within 6" of any friendly model with the Locus of Virulence, that attack inflicts 1 additional damage."
I have a unit of Plaguebearers with an Icon. The Icon is upgraded to a Plague Banner, which states, "When used, increase the Damage characteristic of all plagueswords carried by the bearer’s unit to 2 until the end of the phase."
The PBs are within 6" of a Poxbringer. The Poxbringer's ability adds one to the strength characteristic of Nurgle models within 6".
The PBs are within 6" of a Spoilpox Scrivener. The Scrivener's adds one to Plaguebearer hits, and "In addition, if you make a hit roll of 7+ for a PLAGUEBEARER within 6" of one or more friendly models with this ability, you can immediately make one additional attack at the same target with the same weapon (these bonus attacks cannot themselves generate any further attacks)."
The Poxbringer casts Virulent Blessing on the PBs, which states, "Add 1 to wound rolls made for that unit in the Fight phase. Furthermore, wound rolls of 7+ made for that unit in the Fight phase inflict double damage."

The PBs charge a Knight. There are 20 of them engaged with the Knight, including the Plagueridden, for 21 attacks. What are all the things going on here??

I think it's like this:
The PBs now hit on natural 3s (thanks Scrivener). Each natural 6 grants another attack.
Their Plagueswords are D2 for this phase (thanks Icon). They also reroll wounds (thanks Plagueswords). They are S5, so wounding on natural 4s (thanks Blessing) rerolling against a Knight.
1-3 do no damage.
The 4s count as 5s and so do 2D = 2 damage per unsaved wound.
The 5s count as 6s, and so do 2D + 1D additional (for the Locus) = 3 damage per unsaved wound.
The 6s count as 7s, and so do 2D * 2 (for the Blessing) + 1D additional (for the Locus) = 5 damage per unsaved wound.

So I think what it boils down to is that, when buffed like this, PBs will do 3D on each natural 5 and 5D on each natural 6. Any other natural number that can wound will do 2D.

EDIT: I posted all this to check and see if I had it correctly. Can anyone confirm or correct?

From the Chaos Daemons FAQ:

Chaos Daemons FAQ wrote:Q: If a unit of Plaguebearers has had the Plague Banner Stratagem used on them, and is currently under the effects of the Virulent Blessing psychic power, what damage do their Plagueswords inflict on wound rolls of 7+?
A: 4.

So yeah, you have the correct results there for natural wound rolls of 4, 5 and 6. As far as I can tell the rest of what you've said is correct as well.

I'm not sure if you do (2+1)*2 or (2*2)+1.

But I KNOW that 4s deal 2 damage, 5s deal 3, and 6s deal 5 or 6.

I think natural 6s do 5 damage because the Locus says, "Deal one additional damage." So what's one more than 4?

Yes that is all correct. The next part is keeping track of it all in the heat of battle but after a few games you should remember all the interactions quickly enough.

Also, if you get a DP nearby you can reroll the 1s to hit.