This spell is an exception to the normal spell-control rules[1], allowing the mage to add a Wall to a pre-existing Ward. He can also combine this spell with a Ward during casting[2].
When combined with another spell (which can be provided by a separate caster), this gives the Ward offensive capabilities. For instance, it could be combined with "Evil Eye" to cause those who passed through it to suffer penalties per that spell.
A Ward altered in this manner uses the casting pool for the supplementary spell when it activates. The Wall has one charge of this spell, but additional charges may be applied without additional rolls if done at the time of casting, at a cost of one Mana each (in addition to any normal Mana costs for that spell). These additional charges do not go off all at once; they activate one at a time, as people come in contact with the Ward.
However, this kind of warding is very fragile. Dispelling either the Ward of the encased spell will eliminate both, with the encased spell having an effective Potency equal to the remaining charges.
Example
Pardy has cast Walls of the Labyrinth with four charges of "Celestial Fire" (Prime 4 version) built in. This costs him a total of four Mana (one for the first casting, and three more for the additional charges). While his Ward has a Potency of 6, the Walls portion has only an effective Potency of 4.
- When Bob comes into contact with the Ward, he will be "attacked" by it using Pardy's "Celestial Fire" dice pool. Bob survives the assault and decides to Dispel the Ward/Walls combined spell. After a few minutes of analyzing the spells, he chooses to go after the Walls portion of the spell. He needs only three successes to tear down the Walls (though it he'd noticed them before being attacked, it would have been four successes; one point of effective Potency was used up in the attack).
