The above options only allow you to use a Tenebrium weapon without catching Rot. You can still use one without those skills, as long as you drop or destroy the weapon once you're done and use a Bloodstone to cure your party.
You can buy Tenebrium weapons from the blacksmith in Sacred Stone. Alternatively you can get Tenebrium Ore (Luculla Mines, steal from the Troll King, steal from Sacred Stone), use Smithing 4 to create a Tenebrium Bar (Tenebrium Ore + Furnace) and Crafting 5 to combine the bar with a weapon.