Tarachnid Creeper

Before crafting, you must unlock the Protoform Synthesis Forge by completing the first three chapters of the Secrets of the First Ones campaign, then researching at the cypher console in Exile's Hollow, and completing the questline starting at Finding Tahli. The following day, a new questline becomes available that leads to unlocking Protoform Synthesis.

Travel to Endless Sands in Zereth Mortis and enter the building at 62.85, 22.01 to find the Schematic: Tarachnid Creeper lying on the floor. The schematic only appears after the rubble blocking the entrance is removed during the Aggressive Excavation quest as part of the main Zereth Mortis questline — if the schematic is not present when you arrive, try server-hopping by joining a Zereth Mortis group via the LFG tool.

Gather all three required crafting materials: 450 Genesis Motes, which drop from enemies throughout the Zereth Mortis zone at 70, 28.6; one Tarachnid Lattice, which can be purchased from the Auction House or farmed from specific mobs listed in the item's dropped-by section; and one Revelation Key, which drops from the Protector of the First Ones rare. To reach Protector of the First Ones solo, mount to a lethal height just outside the boss door, fall to your death, then release and corpse-walk back inside the cave as a ghost.

Bring the Schematic: Tarachnid Creeper and all three reagents to the Synthesis Forge at 31.5, 50.3 in Zereth Mortis and craft your Tarachnid Creeper mount.