Leer Cast

How does this work? is the 30% reduced damage mean damage taken or your dps? is it multiplicative or additive?

It reduces the damage you deal. It's additive with other increased/reduced damage mods.

The big question for me is how Leer Casts stack. If you had a party of 6, all wearing Leer Cast, would that mean they each did 60% increased damage (6x15% for the party effects minus 30% for the individual effect)?