Vial of Red Goo
Source: Drop. The event takes 125 Drained Blood Crystals to complete if doing solo.
The 
Vial of Red Goo is a toy dropped by Remnant of the Blood Moon during the Blood Moon event in Tanaan Jungle.
Guide awaiting review
Wait for Blood Moon event
The Blood Moon event must be active in Tanaan Jungle. A herald will announce when the event begins.
Collect Drained Blood Crystals
Once the event is announced, travel to Zeth'Gol and pick up 
Drained Blood Crystals that spawn around the area.
Drain the Blood Moon
Bring the collected crystals to the blood altar in Zeth'Gol village and use them near the Blood Moon. Each crystal drains some of its health. You will need approximately 125 crystals to fully deplete it.
Defeat Remnant of the Blood Moon
Once the Blood Moon is fully drained, Remnant of the Blood Moon will spawn at the altar. Defeat it for a chance to obtain the Vial of Red Goo.
Tips
The event takes considerable time to complete, so you will have time to travel to Zeth'Gol after the announcement goes out.
