I did an early test showing that it was a multiplicative. After being confirmed in discord that it was supposed to be additive like/with other “enemy take increased damage” I decided to do a much better test focused on Undead Harvest (+5% increased damage taken). My better test again returned multiplicative. Not too sure what to think of that, I kinda assume my tests are wrong, but figured I’d report it.
So without undead harvest you would do 100 damage to a target but with the node you’d do 105? There aren’t many sources of “increased damage taken” (Mark for Death being the most obvious one), but presumably if you used both undead harvest & Mark for Death you’d get 136.5 damage dealt instead of 135 damage.