"Serpents' Ire" is a meta event in Guild Wars 2 that takes place in the Branded eastern part of Domain of Vabbi.
This is a short guide how to successfully finish the event:
» First you have to find and kill veteran or higher-ranked Branded monsters to raise the ire of Branded Forgotten zealots and lure them out of hiding.
» Then the players should split into 5 groups, one to each corner to cover and kill each Branded Forgotten Zealot quickly. They should also equip CC skills, as the Zealots regain their defiance bar in 10 seconds.
» After that, 2 Champions will spawn. Players should focus the Forgotten (Ysshi Hessani) first, because if the hydra (Pek Rakt Grag) dies before him, the Forgotten will resurrect it. Also if sparks are spawning, they should be killed quickly as they are healing the boss.
Event description on wiki: https://wiki.guildwars2.com/wiki/Serpents'_Ire