Highlander is a competitive 100 card singleton format of Magic the Gathering. The reason why the term „competitive“ is mentioned here is because Highlander is played 1 vs. 1, each player starts with 20 life points, and in tournament play you have 60 minutes to win a best-of- three. 100 card singleton means that each deck must contain at least 100 cards and only one copy of each card other than basic lands can be included in the deck.
Cards legal for deck construction are essentially all printed, tournament legal cards produced by Wizards of the Coast except for the cards listed on the Highlander Banned List. All cards from new tournament legal Magic expansions become legal on the worldwide release date of the edition.
The playing rules are the same ones as the competitive Magic the Gathering rules by Wizards of the Coast and you can look them up using Google.
THERE ARE HOWEVER SOME RULE EXCEPTIONS:
There is no sideboard because it would ruin everything especially the needed play time.