overlord

Short definition from Wiktionary

Noun

1. 1
   A ruler of other rulers.
2. 2
   A feudal lord directly owed rent or personal service by a tenant of subinfeudated land, a tenant's liege lord.
3. 3
   Anyone with overarching power or authority in a given domain.

Verb

1. 1
   To rule over; to domineer.