This now gives 350 reputation. Also you get 25 rep for killing one of the dukes.

Process is as follows:

STEP 1:
Obtain 4 sets of Twilight Trappings https://vanillawowdb.com/?itemset=492 https://vanillawowdb.com/?itemset=492 (head, shoulders, and chest) by killing the any of the humanoids at the twilight camps. Drop rate on each piece is 4%, so on average you will get one set every 25 kills. That's ""on average"", so expect to have lucky streaks and unlucky streaks.

STEP 2:
Wear a set of Twilight Trappings. Click on a LESSER WIND STONE. A dialogue box opens with only one option. Before you click on it, equip you normal head, shoulder, and chest gear as you will soon be fighting. Click on the dialogue option. In a couple secconds, a flash of lighning will strike you (does no damage) and one set of Twilight Trappings will disapear from your inventory. A Templar will appear at the stone. For about 5 seconds it will just sit there and can't be attacked yet. Then it goes aggressive on the person who did the summoning. Kill the Templar, and loot the Abyssal Crest.

STEP 3:
Repeat step 2, and then repeat it again. You should now have 3 Abyssal Crests, and you should still have one set of Twilight Trappings.

STEP 4:
You will now need 1 Large Brilliant Shard, and you must be friendly with Cenarion Circle to continue.
Do the ""Aurel Goldleaf"" quest from Huum Wildmane if you haven't done it already. Complete that quest by talking to Aurel Goldleaf. For the follow on quest, give her the 3 Abyssal Crests and the Large Brilliant Shard, for which she will give you 1 Twilight Cultist Medallion of Station.

STEP 5:
Go back to one of the Twilight camps and find a WIND STONE (not a ""lesser"" and not a ""greater""). Clear the area around the stone. Put on a set of Twilight Trappings and also put on the Twilight Cultist Medallion of Station. Click on the Wind Stone. You will see a dialogue box, just like in step 2. Switch back into your normal gear. Click on the dialogue option. One set of Twilight Trappings and the Twilight Cultist Medallion of Station will disapear from your inventory, and one of the 4 DUKEs will be summoned. As before, there is a 5 second pause before the Duke will go agro. Kill the duke and loot the Abyssal Signet. Note, if you fail to kill the Duke, or it evades for any reason, it will despawn and you have to start all over from step 1. You do not get any of the materials refunded to you.

STEP 6:
Go back to Bor Wildmane and turn in the Abyssal Signet for your reward.

REPEATING FOR MORE TURN INS:
You can turn in more Abyssal Signets to get more rewards, but each turn in after the first requires 3 Abyssal Signets. You have to repeat steps 1 - 5 for each Abyssal Signet, so that's 3 full repetitions to get another Sack of Spoils.

Is it worth it?
My first time doing this was solo as a 70 BM hunter. The randomly summoned duke was the Duke of Shards, who I downed with no trouble. The duke dropped Abyssal Plate Legplates of Restoration https://vanillawowdb.com/?item=20671 https://vanillawowdb.com/?item=20671
I turned in the quest, opened my Sack of Spoils and got Lord Alexander's Battle Axe https://vanillawowdb.com/?item=13003 https://vanillawowdb.com/?item=13003
From wowhead's loot tables:
Getting a BoE blue drop from a duke is about a 25% chance.
Getting a BoE blue drop from the Sack of Spoils is also about a 25% chance.
It can be a fairly time consuming process to go through and the odds of getting nothing are fairly high, so weigh the cost before you set out on this venture.

All of the dukes have been significantly weakened in patch 2.3. They went from a decent 5-man at level 60 to an easy 2-man at the same level. They do less damage now and went from their 50k+ hp down to a measly 15k.