From Diablo Wiki
Blighter is a Rift Guardian based off the Herald of Pestilence enemies found in Act III. Like a normal Herald of Pestilence, Blighter has a AoE poison attack where he becomes rooted in place during the cast. When Blighter's health drops below 25%, this attack changes so that the number of AoE circles increases drastically. If the player is fighting in a narrow space, he/she will quickly be overwhelmed by the overlapping AoE.