"If opponent is making a rescue attempt and has access to a lost soul in this site, you may discard this site to protect all Lost Souls from rescue by a Hero this turn."
I like this better:
"If opponent's hero has access to this site, you may discard this site to protect its lost souls from rescue."
(and yes, that's "its" not "it's".)
I'm assuming that site access only makes sense in battle. Anyway this seems cleaner.
Protecting all lost souls from rescue is a bit too powerful as we already have a lot of that.