Wraithstrike

You touch and transmute the weapon used in the spell's casting. Until the spell ends, when you make a weapon attack using the weapon, if the target wears nonmagical armour or none, you treat their AC as 10 + its Dexterity modifier.

Magic armour and magic effects that increase AC can't be bypassed with this spell. This includes Mage Armour, Greater Mage Armour, Shield spells or similar.