Materia Cave

There are four caves scattered across the world (one not on the map) which are only accessible by special chocobos. Inside each is one special materia which you will never find anywhere else. The four materia are: HP<->MP, which switches the HP and MP max, Mime, which copies the last attack your party performed. Quadra Magic, which allows the paired materia to be cast four times in a row, and Knights of the Round, the ultimate summon spell. It is cummulative, unlike other summons, and doesn't have an element. Being cummulative, it can deal far more damage, and although some elements would cause more damage, they can also be absorbed or nullified, which can't happen to KotR.