permanent monster spawn is only use in Quest Script
since you say Event Script, you should be using *monster script command instead
https://github.com/HerculesWS/Hercules/blob/master/doc/script_commands.txt#L5599
prontera,150,190,5 script kjdsfhskdfj 100,{ monster "this", -1,-1, "--ja--",1096,1,strnpcinfo(0)+"::Onaaa", rand(3),0; end;}