This is you. If you lose this unit, you lose the game. So don't.Leader is a unit loss of which means loss of the game. In most rulesets Leaders do not appear by default, but one can be included in starting set of units.
--Classic ruleset description
If you kill your opponent's Leader, all his civilization just evaporates the next turn without even ruins of the cities, leaving to you only spare territory and infrastructure (but his cities originally founded by you are returned to you). So taking a city where the Leader resides will leave you with this city only, i.e., fast victory won't bring you much points.
Some other specific features of a Leader include (mainly having Classic ruleset in mind):
- Leader has a SuperSpy flag. Any Diplomat or Spy will always lose a diplomatic battle against a Leader. Also, Leader is, obviously, unbribable.
- Being initial unit, Leader naturally has no home city. But even if health loss of homeless units is turned on in the game, this does not affect Leaders.
- Leader is a non-military, non-attacking unit which ignores ZOCs and has no one himself.
Leaders' behaviour is changed in Webperimental ruleset where they are default starting units. Killing enemy's Leader, you get some of his cities, gold and knowledge, while other cities are turning barbarian. Also, he can heal lightweight units (including himself) for 25 % health. Also, he can raze his own city.
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