Humm I will try it out.
Get it from those dark wraiths mobs?
Reading weapon upgrades/info is a pain on the wiki - I am still clueless.
Gems and infusions are basically how you scale weapons with your stats so you get better damage out of them. Without the scaling a Raw or Fire infusion will typically be better.
Some special weapons cannot be infused, basically telling you that those weapons are going to scale how they're going to scale and thus can't be changed to fit your character as much.
Depending on your stats you want a different infusion for your weapon.
Crystal/Simple Infuse and improve based on INT
Blessed/Lightning based on FTH
Heavy based on STR
Sharp based on DEX
Refined for STR/DEX balance
Fire/Raw to remove scaling and give the weapon pure damage (not based on stats)
Dark/Chaos for INT/FTH balance
There are a few others as well but for now that should help you get something that suits your needs. Basically just pick or find the gem that most matches your stat loadout (so if you're putting points into STR, you want to infuse your weapon with Heavy gems, etc.)
Looks chaotic as fuck. Btw, do you lose souls if you die as a MM invader?
You always lose your souls if you lose/die as an invader, and have to go find them again where you lost them, or risk losing them permanently if you die again.