Originally Posted by Sarpati
That's not what I meant. I mean, the League's magic keeps them from permanently dying. If they were to fight outside of the League, could they die?
In a word: yes.
It is the power of the nexus - shaped by the League's most powerful magicians - which allows a champion to resurrect countless times in a battle arena. Were Kayle and Morgana to square off outside a Field of Justice in a battle to the death, that death would be real.
A real death, however, isn't necessarily a permanent one. Runeterra virtually overflows with raw magical power. A rite of resurrection requires a magician of notable potency to execute, though enough know how to perform this rite that performing it isn't an uncommon act. Necromancy - another means of bringing back a fallen individual - takes a lot less effort and can yield "similar" results.