Well technically he is immune until he opens his chest to call upon the darkness aka the worm, which is what provides him power and immortality. We are essentially killing the worm or damaging it to a point where it no longer finds Oryx worthy and then consumes him.
English
-
Not to mention that we don't damage him, the orbs of corrupted light do
-
Exactly. We simply stun him (also to prevent him using the darkness to kill us) long enough to detonate the corrupted light, which is what does the real damage.
-
True...the process though, is very 20th century though.