Thranconia

Any time a fighter-classed character drops an enemy to 0 hit points or below in hand-to-hand combat, he may make a bonus melee attack against any opponent in reach. He may repeat this to a maximum of 1 extra attack per experience level or 8, whichever is lower. Ogres, Giants, Demons, Angels,Trolls and Vampires can also make such bonus attacks (HD equaling levels).