Hiashatar is a medieval chess variant deriving from Mongolia. The pieces move as in orthodox chess, with the exception of the additional piece, the Bodyguard (also called 'Senior Adviser' or 'Warrior'). It slides one or two steps in any direction. The Bodyguard has about the same value as a rook + two pawns (my estimate). A Bodyguard cannot checkmate (or capture) the enemy king. The Bodyguard can stymie the movement of enemy pieces, except the horse. This implies that an enemy piece can only move one square at at time so long its movement occurs on the squares immediately surrounding the Bodyguard. The horse, being so important in Mongolian life, is not affected by the stymieing powers of the Bodyguard. The pawns move as in orthodox chess, inclusive of the initial double-step and 'en passant'. The pawn can also make an initial triplicate step, which means that 'en passant' can also be performed when the enemy pawn stops on the rank behind the friendly pawn. Pawns promote only to queen. Castling does not exist. The positions of white king and queen are interchanged (compared with standard chess). In Hiashatar, the restrictions for checkmating, which are characteristic of Shatar (8x8 Mongol Chess), are absent.

Thanks to its stymieing capability the Bodyguard is immune against long-range attacks from enemy queen, rook, and bishop. This also means that these pieces cannot easily guard friendly pieces being threatened by an enemy Bodyguard. This makes the Bodyguard a very useful attacking piece in the middlegame. If the Bodyguard is placed centrally during the middlegame, it effectively stymies the long-range enemy pieces. The horse, however, remains a serious threat to the Bodyguard. The Bodyguard's inability to threaten the enemy king is a serious deficit. There is no castling in Hiashatar, and removing the king from its exposed position in the centre is not always necessary, thanks to the effective protective capability of the Bodyguard. In an additional variant the Bodyguard stymies also friendly pieces, and can capture only the nearest square. This is possibly an authentic historical variant.

The word 'Hiashatar' means 'Bodyguard Chess' (Shatranj). This game is still played in Mongolia, although Fide-chess is taking over more and more. Hiashatar is said to have appeared 500 years ago.

 
References

http://www.geocities.com/kisslook/eng/mongeng.html
http://history.chess.free.fr/hiashatar.htm