Currently this rune only works at specific angles when mobs are in a straight line. This only happens when you're completely surrounded in a large aoe but almost never when you have multiple enemies or a medium pack (think south fi, gp, giran castle dungeon).
Suggestion to improve this rune: make the rune cleave instead, even if it is only 1 additional enemy instead of 2. This way it will be useful in more situations.