From Destinypedia, the Destiny wiki
The Plaguewielders are encountered during the Enthymeme mission and encounter. They have an immunity shield making them immune to damage. To damage the Plaguewielders, players must kill hordes of Scorn enemies to lure out Elite Mephitic Host. Killing the Mephitic Hosts will allow the player to gain the Ephemeral Virus, allowing you to damage the Plaguewielders for 29 seconds. However if the timer reaches zero you will be killed. To avoid this, players must jump on the well before the timer reaches zero to cleanse themselves of the Virus. Once all three of the Plaguewielders are slain the encounter is complete.