Overcasting

Clients that draw their magic from a patron are limited by their physical ability to contain the magic.  More experienced adventurers can harness more of the divine energy, as their bodies have built up a tolerance, represented by their spell slots.
Unfortunately, this direct connection means that moments of great stress are echoed along the bond, in both directions.  Extensive damage done to the patron may reduce the client’s ability to cast spells, and traveling to another universe may sever the bond altogether.  In moments where the client is under extreme stress, they may, consciously or unconsciously draw more from the bond than their bodies can hold, inflicting 1d20 permanent damage (unavoidable) on themselves for each level of the spell that they cast beyond the limits of their reserves.
This overcasting is legendary, as it can save a party from the brink of defeat, or bring back people that would otherwise be unrecoverable.  There are a handful of tales of martyrs from the past that have used this ability to halt great tragedies, often at the cost of their own life.