The antiking (represented by an upside-down king) must always remain under attack. It can capture pieces of its color.

Win by checkmating the king, or anti-mating the antiking — that is to say, making it unable to move to an attacked square.

In Anti-King I,

• Pawns are Berolina: they move diagonally and capture vertically.
• King and Antiking can play one non-capturing knight move, as long as they never moved earlier.

chessvariants page.

Peter Aronson (2002).