Once every decade on the eve of St. Poskov's Day during mid-winter, the coastal city of Tiyabon experiences a horrific event. Quool's Tide rolls in, depositing hundreds of bloated, fish-eaten corpses upon the pebbly shores of Tiyabon's wide bay. This singularity is to this day unexplained, though countless theories abound. It is said for example, that these corpses are not eaten by the myriad fish of the seas completely, due to the fear all creatures of the seas hold for Quool.
Named for Quool, a terrible, antediluvian god of seas and storms, who no longer exists for he has no worshipers, the Tide chokes the beaches and surf with the countless rotting bodies of those who had perished at sea in a violent way.
Almost immediately, the lifeless corpses are fed upon by crabs, gulls, and worse things that await the horrid feast. The townsfolk let nature take it course with disinterested disgust, though lately some enterprising adventurers have taken to searching along the beaches of flesh for former deceased companions, with intentions of raising them again!
Surprisingly no undead ever rise from among the many corpses. This is also a mystery.
Rather than making a baddy impossible to hit, consider giving it mega-hit points.
Super hard to hit leads to great player frustration. Allowing them to do damage leads to a sense of accomplishment. another variation of this would be to give the bad guy a forcefield that shows damage.You could also give them a mastermind ability that sluffs the damage of to minions who should be easy to kill.Once in a while a super high to hit encounter is ok ,but it should be few and far between.
Your mileage may vary