[thejeff]

As I suggested above it's a very nice early counter to an SC pretender. It's Thaum 1, so easily reachable within a few turns of being attacked, even if you didn't have it already.
And pretender SCs in the first year or two don't usually have the gear researched or forged to handle even regular horrors.

Other astral counters are much higher research or ineffective against SCs.

It's like curse. Sure, it won't kill you right away, but it's a good deterrent. And unlike curse, it does get worse with more castings. 4-5 mages scripted to 5xHorror Mark, retreat with enough chaff to delay.

Again, mid-late game there are better options and being Horror marked isn't so deadly, but early on it's scary.