Is it permissible for a person to title himself "the Ruler"? Thank you.

There is no objection to titling a person as "Al-Hakim" (The Judge/Ruler), which is an active participle for one from whom a ruling has been issued. Among the scholars of Hadith, it is applied to one who has memorized all that was narrated from the Prophet, peace and blessings be upon him, whether sound, weak, or fabricated; or to one who has memorized eight hundred thousand Hadiths with their chains of narration and texts, along with knowledge of the conditions of their narrators. This title was bestowed upon Imam Abu Abdullah Al-Hakim, the author of Al-Mustadrak.