Despite how it may appear in the index of monsters found in this BCNM, there are not 5 Skeletons. There is just 1 Skeleton which uses reraise 4 times, meaning it has to be fought 5 times in a row. The first 3 times, Ghul-I-Beaban is a DRK, the last 2 times, it is a BLM.
Each time the Skeleton reraises, it starts out with 10% less HP than the one before it, starting at 100% with the original Skeleton.
The melee attack power of the NM, along with it's size, increases with each new incarnation.