During Combat:
If he sees that the frost worm is nearly dead and surrounded by enemies, he'll attempt to kill the worm with his corrupting touch in order to trigger its death throes, knowing that only the supernatural piercing damage can hurt him (and even then, at best, he'll only take half damage from that, due to his incorporeal nature.
My question is he dosent have any Defensive Abilities like immunities but in during combat is "only the supernatural piercing damage can hurt him" so what it is with him?
Yes but in during combat section writes "knowing that only the supernatural piercing damage can hurt him" so it points that he is immune to all injuries except magick pircing How did You understand that?
No, the phrase "knowing that only the supernatural piercing damage can hurt him" is specific to the Frost Worm's death throes. Either as a result of his own actions or those of the pc's.
Karivek is immune to cold and incorporeal - therefore no damage from the 12d6 cold and half damage from the 8d6 piercing damage (which is supernatural.) [See incorporeal description: It can be harmed only by other incorporeal creatures, magic weapons or creatures that strike as magic weapons, and spells, spell-like abilities, or supernatural abilities.] Karivek will take damage from magic weapons and spells, etc. Given his fire vulnerability that might be a really good approach.
Did that answer your question? It's not clear what you're asking since the answer seems to be contained within your two posts.
Yes, thx for answering.