Crogon said:90% of all 'effects' that affect monsters, have no effect on bosses. A few of them, such as Attract do work. Any damage modifiers such as Lower Resists, and those that reduce enemy armor class, do work. Not many other things do. There is an entire strategy guide about this in the strategy compendium. Or at least there used to be one, before they re-vamped it. Your best resource would be to check there and see what it says. Note that the Eth was bugged in 1.09, and reduced an enemies armor class TO 25% of its original value, instead of subtracting 25% FROM an enemies armor class. This made the Eth rune quite desireable, even in elite weapons like a grandfather. However this bug has been fixed in 1.10, so using an Eth rune in a weapon, especially an elite, end game weapon is MUCH less desireable.