Darn it, theres no need to create a NpC script from zero to make an item change an armor property!!!
Just insert this lines, in the item script : time duration numbers are represented by miliseconds:
180000 = 3 minutes
dark
{sc_start4 SC_ELEMENTALCHANGE,180000,1,Ele_Dark,1,0; },{},{}
shadow
{sc_start4 SC_ELEMENTALCHANGE,180000,1,Ele_Holy,1,0; },{},{}
if you want something different, just type Wind, Fire, Water, Ghost, Poison Etc Etc Etc
if you want add an effect:
just type any special effect, example:
{specialeffect2 91; sc_start4 SC_ELEMENTALCHANGE,180000,1,Ele_Holy,1,0; },{},{}