Abyssus increases physical damage taken, which occurs after damage mitigation[1] when receiving damage. The mod is additive to (span class="tc -mod")You and your Minions take #% reduced Reflected Physical Damage(/span)
The downside (span class="tc -mod")(40-50)% increased Physical Damage taken(/span) can be bypassed by taken physical damage as non-physical damage. But not vice versa.