If I were to take this legacy, based on Ishuli's statement, my
presumption would be that I would either be sometimes or always negating
a small portion of sanctuary's damage resistance. Like...I swing a 100
damage hit on someone with sanctuary. Instead of it now doing 50
damage, I would expect it to do somewhere in the range of maybe 55-60
damage.

In part, I'm basing my total guess on the thought that this is an
existing legacy that already does stuff, so I expect the sanctuary bonus
is unlikely to be a major perk.

But then, I'm also a big proponent of pick what's themically right for
the character, and enjoy the character for who it is and however they
end up!