So far as is known, Gwanwr was first wielded by -- and likely created for -- a skilled assassin who was a high-ranking Bhaal cultist.  This unnamed cultist was defeated centuries ago and Gwanwr was looted from his corpse.  It has since passed through several other hands and is currently rumored to be in the possession of a powerful adventurer.

There are two different versions of Gwanwr, one with a magical enchantment of +3 and one with a magical enchantment of +5.  The latter is intended primarily for characters who have progressed beyond level 12 by use of mods.  Install only one of these versions as they will overwrite one another.

In addition to a magical attack bonus Gwanwr has the following special abilities:

Lethal Weapon: Ignores resistance to Piercing damage.

Improved Critical: The number you need to roll a Critical Hit is reduced by 1 (can stack).

Gwanwr's Aura: At the start of combat or upon the wielder entering turn-based mode, Gwanwr will radiate an aura which is identical in effect to the Aura of Murder radiated by Bhaalist Armor: enemies within 2m become Vulnerable to Piercing damage.  Normally limited to NPC Bhaal cultists or to a player who accepts service to Bhaal and becomes an Unholy Assassin, this is an extremely powerful ability which effectively doubles the weapon's Piercing damage. An advantage of Gwanwr's Aura over the armor's Aura of Murder is that the former is inactive except in combat, so NPCs don't flee from the character wearing Bhaalist Armor.  Gwanwr is neither good nor evil and is usable by any character regardless of what path a player chooses; Gwanwr potentially grants a player who chooses a "good" path access to a powerful ability normally restricted to players who choose to side with evil.

Needle Sharp: The point of Gwanwr's blade is preternaturally sharp, granting Gwanwr 1d6 of bonus Piercing damage.  This is effectively doubled to 2d6 by Gwanwr's Aura (see above).  This bonus can be considered equivalent to the elemental bonus damage (fire, cold, lightning, radiant, psychic, necrotic,etc.) that many other magical weapons deliver.

INSTALLATION

Installation is as for any other PAK file mod.  I install all of my mods manually, and if you choose this method you'll need the information below:

<node id="Module">
<attribute id="UUID" value="c1c4f4e9-bfb7-4cd2-aac8-2d992291ffce" type="FixedString" />
</node>

<node id="ModuleShortDesc">
<attribute id="Folder" value="Gwanwr" type="LSString" />
<attribute id="MD5" value="" type="LSString" />
<attribute id="Name" value="Gwanwr" type="LSString" />
<attribute id="UUID" value="c1c4f4e9-bfb7-4cd2-aac8-2d992291ffce" type="FixedString" />
<attribute id="Version64" value="36028797018963968" type="int64" />
</node>

I don't use either Vortex or BG Mod Manager, so I cannot guarantee that this mod will work with either of those but I don't see why it wouldn't.

FINDING IN GAME

Gwanwr can be found in the tutorial chest in the nautiloid.  If you are already in a playthrough you can summon the tutorial chest using wesslen's excellent Tutorial Chest Summoning mod.  If you are using Laughing Leader's Script Extender with the console enabled you can spawn Gwanwr using the console command below:

Osi.TemplateAddTo("c8e3db0d-e5cc-4ead-a49e-c52220937dd8", GetHostCharacter(), 1, 1);

Alternatively, you can use AnteMaxx's excellent Spawn Any Item mod, using UUID c8e3db0d-e5cc-4ead-a49e-c52220937dd8.