Barndisglair (Radiant Doom) was first wielded by Tânaur, a paladin of Torm who lived several centuries ago.  It is a beacon of light and hope which drives back the darkness and is a potent weapon against evil in any form but especially against fiends, undead, and creatures of shadow.

Barndisglair has a magical attack bonus which ranges from +1 to +5 and inflicts bonus Radiant damage which ranges from 1d4 to 2d6 depending on the wielder's character level.  The attack bonus and bonus damage are:

Character Level 1-4:  +1 attack bonus, 1d4 bonus Radiant damage
Character Level 5-8:  +2 attack bonus, 1d6 bonus Radiant damage
Character Level 9-13:  +3 attack bonus, 1d8 bonus Radiant damage
Character Level 14-19:  +4 attack bonus, 1d10 bonus Radiant damage
Character Level 20+:  +5 attack bonus, 2d6 bonus Radiant damage

Note that this progression supports character levels above 12, achievable with certain mods.

In addition to a magical attack bonus and bonus radiant damage it has several other abilities which enhance its effectiveness in combat:

Lethal Weapon: Ignores resistance to Slashing damage.
Improved Critical: The number you need to roll a Critical Hit is reduced by 1 (can stack).
Whirlwind Attack: Strike out at all nearby foes, making separate attack rolls against each target.
Blinding Radiance: This is identical to Lathander's Light provided by the magical mace Blood of Lathander.
Revealing Light: This combines the effects of the Daylight and See Invisibility spells and is automatically activated and usable only in combat.

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="81334b92-be5d-4b81-8061-b1fc26569168" type="FixedString" />
</node>

<node id="ModuleShortDesc">
<attribute id="Folder" value="Barndisglair" type="LSString" />
<attribute id="MD5" value="" type="LSString" />
<attribute id="Name" value="Barndisglair" type="LSString" />
<attribute id="UUID" value="81334b92-be5d-4b81-8061-b1fc26569168" 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

Barndisglair 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 Barndisglair using the console command below:

Osi.TemplateAddTo("8df0ff98-49f0-4c3d-9f38-6fa006814580", GetHostCharacter(), 1, 1);

Alternatively, you can use AnteMaxx's excellent Spawn Any Item mod, using UUID 8df0ff98-49f0-4c3d-9f38-6fa006814580.