Weaken Defenses (Larkins Spell Book)

When cast, this spell weakens your enemies defenses.

Weaken Defenses: Ob3^, 3 Actions
Origin: Presence
Area of Effect: Presence
Element: Earth
Impetus: Tax
Duration: Sustained
Resource Points: 12

Meeting the obstacle lowers the effectiveness of armor and shields by one die (-1D). Exceeding the obstacle reduces them further (-1D per additional success).
Weapon Length: as Spear (longest)
Range: as Thrown (1D/1D) 30 Paces

Spell is only effective against manufactured (“Earth” based) armor.

(This is the type of spell a mage would cast to aid his companions in battle.)

No Sigils, distilled origin with area, and element with duration (pretty straight forward abstraction).

Any tips on how to describe this in-story? What would the effect of the spell look like? What would it look like when someone’s weapon hit someone who’s armor had been reduced to 0D with this spell?

The armor deteriorates for the duration of the spell, how badly depends upon how many dice were lost. When the spell is over, the armor returns to its pre spell condition, minus any combat damage it sustained.
The visual effect would vary with the type of armor but they just look rusted and busted, as if suddenly aged.