Synchronous Thread

[item=168129] is a drop from Dazar in Kings' Rest (last boss)
[item=168128] is a drop from Rezan in Atal'Dazar (the dinosaur, technically third boss but most people kill him first)
[item=168130] is a drop from Viq'Goth in Siege of Boralus (last boss)
[item=168131] is a drop from Avatar of Sethraliss in Temple of Sethraliss (last boss)

These can be done on normal, heroic or m+0 difficulties to get the drops.

[item=164733] is required to access Time Rifts around Zandalar and Kul'tiras. It is gained via the recipe in the Tools of the Trade quest line for Tailoring which requires 150 in Kul Tiran Tailoring to unlock it.

First and foremost unlike other profession tools you have to actually click the item in your bag to interact with the time rift, then it will channel for 1-2 sec.
Active Time Rifts will show up as a big yellow dot on your mini map, but look like swirling yellow vortexes similar to during the quest line.

You can receive [item=168162] which gives some cloth from a random expansion. It can summon a Chronal Anomaly which gives you a grey item worth a few gold. It can also summon a Time-Lost Trader which sells rare old world tailoring recipes for between 25-150 Embroidered Deep Sea Satin depending on the quality of the item.
Personally, I have found they have a 2-5min respawn timer. I haven't found there to be any diminishing returns farming them, I had the vendor appear at 2 out of 7 rifts.

Here is a list of locations I have found time rifts thus far and I will endeavor to keep updating it.
I have posted some screenshots which will help with locating them.

Tiragarde Sound
/way 75.42, 12.49 bottom of penguin sledding zone in the mountains north of Boralus
/way 74.73, 45.65 Eastpoint Station on a rock at the end of land spit north of Bridgeport
/way 76.39, 62.20 on a rock north (downhill) of Kennings Lodge flight point
/way 84.82, 76.41 on the wharf down the hill from Freehold summoning stone
/way 53.64, 59.61 on a rock on in the middle of the "G" in Vigil Hill, looking toward the inlet just south of Hangman's Point Drustvar

Drustvar
/way 60.26, 21.07 right hand side of Barbthorn Ridge
/way 52.08, 23.07 North Pass Caverns next to first small pile of rocks as you come up the hill into the rock circle open area
/way 33.23, 23.28 Corlain Aviary in some trees between the large building and the small crypt to the north

Zuldazar
/way 43.66, 77.40 Xibala on top of the hill above the Dark Chronicler.
/way 44.86, 39.00 along the rocks just south of Atal'Dazar summoning stone

Nazmir
/way 57.63, 42.97 The Heart of Darkness, directly west of spawn in from Boralus near some ruins
/way 43.70, 29.28 Necropolis just at the east entrance

The Tools of the Trade: Tailoring Questline starts at Kul Tiran or Zandalari Tailoring Skill 150. Upon relogin into the game or leaving the area and returning to the trainer, next time you will see Timeweaver Delormi near the trainer. She will provide the quest titled A Friend in Needle.

1. A Friend in Needle
2. A Stitch in Time
3. Tearing at the Seams
4. Rolling Up the Sleeves
5. The Severed Thread
6. Step Into Her Shoes
7. If The Shoe Fits...
8. Seam Stress
9. Saving Nine
10. Killing Time
11. Cut from the Same Cloth

At the end of the questline you will be rewarded: Recipe: Synchronous Thread

Crafting the Synchronous Thread requires the following:

• 
  
• Essence of the Troll Dynasty (1)
• 
  
• Essence of the Regal Loa (1)
• 
  
• Essence of the Depths (1)
• 
  
• Essence of the Sands (1)
• 
  
• Tidespray Linen (100)
• 
  
• Nylon Thread (100)
• 
  
• Expulsom (10)
• 
  

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