I´ve discovered, that 2 events are taking place on the shrine:
https://wiki.guildwars2.com/wiki/Defeat_the_Void_champion after defend the speakers outpost. It has 3 different champions, none counts to this achievement.
https://wiki.guildwars2.com/wiki/Defeat_the_summoned_creature_before_it_can_escape The important event. It is started by this 2 NPCs "Speaker Yuma Silverpinion" and "Speaker Eunjoo" . They go through the shrine and call for help, but they get 1 of the 4 champions. The NPCs are waiting in the speaker outpost and don´t need to get started by a player