The title the Annihilator is earned by defeating all of the nihil while they are attacking you directly, during or after Fate of the Gods. This can be done during the quest or by selecting replay "quest boss fight". Unlocking this title is both a master quest cape and completionist cape requirement: Annihilator.
This title changes according to the language of the server on which RuneScape is played, as shown below:
||[Name] der Vernichter / die Vernichterin
||[Name] l'exterminateur / l'exterminatrice
||[Name] o Aniquilador / a Aniquiladora