Curious Crystalsniffer

Just to refine the location, I personally found it leaning on the right side of the northern pillar in the second phase area of Halondrus

The schematic for this mount is located in the Halondrus boss encounter area in Sepulcher - Can be found by one of the gate things on the room where the second phase of the fight happens. Pattern doesn't spawn until you kill the boss, so you'll need to join a group, kill the boss, and then run back to the second phase's room to loot the pattern.

If you reset the raid in any way, the walls that separate Halondrus's rooms go up again, so this pattern becomes unreachable on that lockout. This is also currently under research, but there seems to be a timer of 10-15 minutes for you to loot the schematic after the boss dies.

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I can confirm this, i looted it, its above the bridge hidden next to a wall at 2nd phase of Harondus after he is killed.