The Depatterning Rating is
a measure of how much stress the use of blood magic has caused
to the character’s True pattern. Each blood magic charm has its
own Depatterning Rating, the amount by which the character’s
rating increases if he uses the item

For every subsequent use of
a blood charm, even those charms with a Depatterning Rating
of 0, the gamemaster makes a Depatterning Test for the character,
using the character’s Willpower step against a Difficulty
Number equal to the character’s current Depatterning Rating. If
the test fails, the character’s True pattern begins to weaken, and
the character’s Depatterning Rating is increased by +1. If at any
time the Depatterning Rating exceeds the character’s Willpower
value, the next time that character uses a blood charm and fails
the Depatterning Test his True pattern dissolves and he dies
spontaneously and horribly. The test is only made on activation
of the charm. For example, if the charm has a Strain cost per
round, the gamemaster makes a Depatterning Test only when
the charm is activated, not each round.

If a character has a blood charm noted in the description
as being susceptible to stress depatterning implanted in his
body, and suffers an injury to the specific part of his anatomy to
which the blood charm is attached (as the result of a Called Shot,
for instance), the character makes an immediate Depatterning
Test. On a Pathetic result, the character dies instantly and horribly
in a manner determined by the gamemaster. Magicians call
this unfortunate phenomenon “stress depatterning.” They don’t
readily admit that it can happen, simply because it depresses
sales of blood charms.