Use this [map] and take a look. Trying to describe where anything is in the Great Crystal (in a short and concise manner) is futile when only using words.

I do not know if it respawns so that a player could conceivably get two Excaliburs or if getting the other treasure in the chest irrevocably ruins your chances of getting the sword, so if you do not get it the first open the chest, reset and try again.