Vekniss Guardians are mobs located in Ahn'Qiraj. When one of the Guardians in a pull is about to die, the remaining guardians charge toward their fallen comrade, impale (stackable DoT), and knock people near the dying mob into the air.
The impale attack of each Vekniss Guardian can be nullified by tanking the current kill target out of line of sight of the rest of the pack. The use of a corner is preferable.