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Venomblade - Sword of the Spider

Venomblade is a sword forged with an enchanted venom-infusing process, and crafted from the carapace of a frostbite spider, leading to it's texture and hardness.

You can find the sword itself on a desk in the boss room of Harmugstahl (locations in slideshow).
For those of you who already cleared Harmugstahl, you can craft it once you have the Steel Smithing perk.
Either way, you need to beat Harmugstahl (condition check that Harmugstahl is marked as cleared), which has one doozy of a puzzle and has to do with Frostbite Spiders.
If you need help with the door, you can read it here;
http://elderscrolls.wikia.com/wiki/Harmugstahl#Lever_puzzle_solution

Venombane can be disenchanted, basically giving any weapon a near-permanent frostbite venom poison coating (recharged via soul gems, not poisons). The sword can also be improved by frostbite venom at the grindstone.

If you have the Elven smithing perk, you can upgrade the sword to Venombane, now with +10 damage to animals and a general damage upgrade, at the forge. Venombane can be upgraded even further at the grindstone, however cannot be disenchanted.

**I recommend getting the ''See Enchants'' mod here on the Workshop, (not totally sure, but I think) it adds a neat dripping venom effect to the sword, not to mention it works on all the vanilla enchants.