Supertransmuting King (Pressburger King): When checked, a king must play as the unit or units giving check if possible, thereafter the king becomes an ordinary (non-royal unit) of this type permanently. If it is not possible to move as the checking unit, another piece may parry the check and the king does not change. If that is not possible, the king is mated. (Stephen Emmerson, A Glossary of Fairy Definitions, BRITISH CHESS PROBLEM SOCIETY, July 2018)