Creating A defend Stance skill

hi, i am trying to make a skill wich will, at first, lower the dmg dealt to the unit by 50%.
since i don't know how to reach that, i decided to just give it X armor.
now my problem is to use the correct spell for it.
if anyone knows a spell wich can give an amount of armor to a unit please say so :]
also the spell it self doesn't have an ending time, this skill is based of defence so it can be deactivated anytime, so i need a good way for adding armor to a unit till the skill is deactivated :]

Edit, ok i made it this way:
i created a knew ability off Devotion Aura,
changed the area of effect to 0, changed to add a 50% more armor,
used triggers to add this to the unit and remove by need.
works perfectly fine, altough, i am using spell books to clean up the unit's skills and abilitys menus,
so it adds it right where all these attack order and move order, i want it to add it self to another spell book. or even not to be seen, is there anyway?

You should be able to use the Defend skill to reduce % damage.

it only reduces dmg from pericing attacks, i tried that

You can use a separate Spellbook for whatever passives you want to hide. Give it a different order string (Spellbook is one of the few abilities that has a Data field that lets you change that) and give it to the unit. Then disable that Spellbook for that player via trigger at game init or somesuch. Passives inside it will still be applied to the unit.

em, the thing is, other abilitys are effected by this passive, i want later to be able to check if a unit has ability (in this case, that passive ability).
all i want is to make another spellbook wich is visual, wich has other passive abilitys, and wich i can add and remove a passive ability, can i add abilitys or remove them in a spell book? how so?

Edit:
i have figured this out,