Issth - The Serpent of Light, dexterousness, trickery.
Na - And/Of
Sariour - The moon, magic, fortune, evil deeds, destruction
Altogether, Issth na Sariour might be roughly translatable as "Serpent of the Moon's Light", to reference how Mathilde is taking on a more sinister or deadly persona that strikes during the night; or possibly "Trickery under Moonlight", somewhat referencing Branulhune; but also possibly, "Wisdom Asp's Fortune", given how it was an irreproducible stroke of luck (its own misfortune) to obtain its blood the way we did. All are meanings that can apply to us, in ways that aren't immediately obvious.