dat1.xml it is self explanatory and there is a ton of tutorials talking about adding items and it is the same for projectiles just copy another one and edit it.
And if you want to sword to shoot the custom projectile just replace Darkness Bolt or whatever is between <Projectile><ObjectId>$$$</ObjectId></Projectile>(This is the sword of Majesty)
Code:
This is for the weapon :
<Projectile>
<ObjectId>Darkness Bolt</ObjectId>
<Speed>100</Speed>
<MinDamage>235</MinDamage>
<MaxDamage>290</MaxDamage>
<LifetimeMS>350</LifetimeMS>
</Projectile>
This is the Projectile :
<Object type="0x0013" id="Darkness Bolt">
<Class>Projectile</Class>
<Texture>
<File>lofiObj2</File>
<Index>0x71</Index>
</Texture>
<AngleCorrection>1</AngleCorrection>
</Object>
Adding the sprite is simple it's just like adding a weapon texture