Krynn the guardian

Harvested from a giant spider from Azilian, only a few doses of this poison still exist today in the world because the only known Krynn spider has been killed, so it will never be recollected. The poison attacks nerve cells directly in a very violent way.
A creature subjected to this poison must succeed on a DC 18 Constitution saving throw or be poisoned. While poisoned in this way the creature's Intelligence score is reduced by 3d6. If the creature's Intelligence score drops to 0, it dies. Only magical healing like the greater restoration spell or other magic can end the effect of this poison.