Subirachs has in fact had something of a helpless feeling about how to guide Sevar towards Asmodeus here; Sevar's earlier punishment in the palace torture chamber, from the Queen, apparently hugely backfired and shook her faith significantly because the torture did not immediately make her stronger.
She will fall back on Rugatonn's apparent strategy; make sure that, if the torture doesn't work, it's at least Sevar's own fault for doing it wrong, so that her Irorian leanings will tell her to torture better next time and not that torture is bad.
Subirachs will ask Sevar what punishment she intended to inflict on Tonia if she failed.