The game's collection data lists Membrane of C'Thun as sold by a vendor NPC in Dalaran.
Vendor
Zone
Map
Waypoint
Other items it sells
NPC #28997
Dalaran
-
-
96
Vendor NPC, zone and position come from the client's collection source tables (CollectableSourceInfo and CollectableSourceVendorSparse); coordinates are the in-game map readout.
Build history
First seen in build 5.5.4.69585 (Mists of Pandaria).
Read from the World of Warcraft client build files (JournalEncounterItem, JournalEncounterCreature, Achievement) and the Blizzard Game Data quest API.
How Membrane of C'Thun Compares
Who It Suits
Leather is the armor type Demon Hunter, Druid, Monk and Rogue wear. With Agility or Intellect, Membrane of C'Thun suits Assassination, Outlaw and Subtlety Rogue; Brewmaster, Mistweaver and Windwalker Monk; Balance, Feral, Guardian and Restoration Druid; Havoc, Vengeance and Devourer Demon Hunter.
Transmog Sets
Membrane of C'Thun is part of the transmog set Wind Dancer's Regalia (Rogue only). The other 16 pieces:
2 other items in the current build are also called Membrane of C'Thun. Same-name entries are usually a different rank, faction, quest stage or reward track of the same object.
Blizzard numbers items as they are created, so the named items next to ID 59490 (59484 to 59495) were usually added in the same batch as Membrane of C'Thun: the same quest line, zone, vendor or reward set.