Travel to Antonica and head towards the Windstalker Village ( -1865, -38, -550 ) /waypoint -1865, -38, -550. Jump into the lake and swim through the underwater tunnel into a cave. Speak to Stephen Pikefist ( -2046, -39, -571 ) /waypoint -2046, -39, -571. Make sure you complete the whole conversation with him before leaving so you can get the wand from him.
Head to Glade of the Coven ( -1544, -14, 24 ) /waypoint -1544, -14, 24 and use the Wand of Putrescence on any mammal. The Putrescent Anathema (level 55 mob, non-heroic) will spawn and attack what you used the wand on. After that creature is dead, kill The Putrescent Anathema and loot the corpse. If the Anathema is grey to you it may not allow you to attack it after it kills and the quest doesn't update if you kill it while it is killing something. If that happens use the wand on another creature then trigger an AoE attack near the Anathema afterwards.