Holy Win is the tenth-Tier ultimate Great Sword; like the Hand of Light Heavy Mace and Rhomphaia Sword, it has unusual properties in Combinations. Like all Great Swords, a Long Weapon, it cannot be used with a Shield. Like all Great Swords, it is Edged Type, and is preferably used with Edged-Type Grips. It is the rarest weapon in the game; although there are four other weapons at the same low drop rate of 3/255, there are more than one instance of them dropping, and/or they drop at higher rates from other enemies as well. At 35 Strength, it is the tied for second highest damage blade in the game with Halberd, behind Destroyer with 36.
Like Rhomphaia and Hand of Light, it imparts its material type to any weapon without changing the Design. In this sense, it is is separate from normal Progressions. Unlike the Rhomphaia and Hand of Light, there are absolutely no exceptions to this; every single combination with a Holy Win Sword and another blade will replicate the other blade in every detail, except it will be the material of the Holy Win
There are only two Holy Win swords in the entire game, both Damascus, and only one drops. It is therefore impossible to have a Holy Win sword in any material other than Damascus. The first one, of course, belongs to the penultimate boss enemy on the roof of the Great Cathedral, and this unnamed sword with a Spiral Hilt never drops.
The other is the "Excalibur" weapon from the Last Crusader enemy that appears in the Tormentum Insomniae room in the Second Iron Maiden if Ashley has 149 HP or less. Excalibur has an Arturos gem, a Spiral Hilt, and a DAMASCUS blade, and drops three times out of 255.
Vagrant Story Insufferable Walkthrough 138: Holy Win! -Sir Rosser, YouTube. At about 12:00, the best strategy (North to the exit, and back in to respawn the Last Crusader) is employed, and a minute and a half later, the Holy Win is obtained (you results will almost certainly vary from this)