What do you call someone who's running after power and is hungry of controlling everything/everyone, all the time? Somehow like
a moderator Darth Vader.
Such a person (as Darth Vader, not an ELU moderator! :) would be called a...
megalomaniac - a person who has an obsessive desire for power
The adjectival form megalomaniacal does occur, but it's nowhere near as common as the noun.