an Immortal Skeletaur
resurrecting since you didn't pull the out the magic sword in time. This skeleton also levels up at that moment.
The magical sword is a powerful legendary artifact held by a Immortal Skeletaur in Tiny Tina's Assault on Dragon Keep DLC
You must pull it out of the Skeleton's back after killing it, otherwise the artifact will cause the skeleton to get back up as well as level and become harder to kill.