A Plague of Burrowers is a special event in Adventure Mode of Diablo III. It can only appear if nominated a bounty, in any zone of any Act. If it appears, one can go through a red portal and access any of the secret zones: Leoric's Gardens, Blighted Sewer, The Killing Grounds, The Archives, The Bound Keep, or Nephalem Proving Ground.
The event itself requires cleansing a Cursed Chest. Once it is touched, a timer will start, and multiple Scouring Lobbers (named Cursed Burrowers) will spawn, any of them, if killed before the timer expires, will spawn an additional one. Once the timer if over, the player must clear any remaining monster, and in the end, kill the Cursed Matriarch. If at least 6 Burrowers are killed before time is up, one will receive a bonus Radiant Chest.