The Fae, they are called, though the reason they are not called by their true name, the Fairies, has been lost to the ages- at least, by the humans. The other races know, the other races know well of the Fae.
Humans have a very short memory. The elves, the dwarves, the goblins, the orcs, they don't. They remember of the interactions between Atheians and the Fae all those years ago in that other age. They know what happened. But the humans... they have forgotten.
This is why they will be the first to die.
A war of ambition threatens the unseelie houses of Thiran Nar.
The blade of the adamant blossom…
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