Hi Go to global settings, toggle Global Damage Modifier on right hand side, now go to Core page, click the gear icon for the newly displayed global damage modifier, give it a name, say "Arcane Armour" put 1d6 in as damage and 1d6 as Crit, now put this in where it says type ?{DamageType|Acid|Cold|Fire|Lightning|Poison|Thunder} and away you go. Spell active, click the box for this global damage mod, spell expired, de-select. Not familiar with spell, but assume it has a duration above one round and also, assuming 5e sheet