Ruler

wiktionary

ref

From Middle English reuler; equivalent to rule +‎ -er.

etymonline

ref

ruler (n.)

late 14c., "one who rules or governs," especially in reference to superior or sovereign authority, agent noun from rule (v.). The meaning "instrument used for guiding a pen, etc. in making straight lines" is by c. 1400 (compare rule (n.), the older word for this).