Breach

Xiphaem is very good at overriding Celestial claims.
'Breach' is Meraxes' name for his creation. Essentially, a rapidly self-replicating xiphaem fragment that explodes into branches of claimed blood once it strikes a target. Its primary weakness is burning out: the stronger the spell, the faster it burns out, causing less trauma. Weaker versions of the spell can fester for longer, allowing more serious damage to be done (the branches exploding outwards having more time to grow), but cannot override claims as easily and have issues with harder substances. It's a similar dilemma to hollow points vs fmj- the former does more damage to soft targets, but is more easily stopped by armor.
Breach functions across loose superimposition, and travels across any planar links that the target may possess. A properly directed Breach can even use conceptual definitions as a bridge, acting in Celestial space. In this way, Breach is a step closer to true soulcraft than practically any other known application of xiphaem. Leveraged properly, it's the ultimate tool of surgical elimination, theoretically capable of destroying anything that the wielder wants, and nothing more. It sees occasional use in exorcism and similar practices for this property: one can create a target with conceptual link to an untouchable entity, and damage it through said link.
It may be odd to think of a blood-based magic working across conceptual lines, when blood is obviously a very physical thing. Xiphaem breach is the form in which the magic is most typically understood, but not the perfect ideal of it. Conceptual barrier breaching is, in fact, the original use of this working: the moment of inspiration in which it coalesced saw it used to strike an entity thought of as outside reality: observer, but not participant, to the cycle of life and death. Adjusting the idea to function on a more physical level was an adaptation to make it more useful day to day, but the working's origins lay etched in its name, and its invocation always echoes, at least a little of, the intent that it was originally crafted with.
Sorcery-breach (i.e. not the xiphaem specialization) is one of the few ways that one can damage a target by attacking its shadow.