Ok, I've made my decision. The Magical weapon that falls in Fulgrim's lap is going to be a home-brew, and sentient, with an alignment that matches Fulgrim's. Here are the stats:
It can speak, read and understand Common, Sylvan and Druidic. In addition, it can communicate telepathically with anyone who holds or wields it.
It has normal hearing and vision out to 30 ft.
It's alignment matches Fulgrim's
It's special purpose in creation was to seek to defend the servants and interests of the Grandfather Tree. It abhors killing living things, regardless of their alignment, but thirsts for the destruction of the undead, fiends, and the like.
I has an Intelligence of 13, a Wisdom of 12 and a Charisma of 12.
It is exquisitely crafted from the finest materials and glows with a pale radiance - moonlight, shedding dim light in a 5-foot radius. Any metal in the item is silver or mithral, rather than iron or steel.
Otherwise, it is modeled after the Moonblades, only Nature centric rather than Elf centric. I rolled that it has 6 runes on it when it comes to you, a 7th appearing once you pick up the weapon. I am rolling these randomly.
Rune 1: Makes the weapon a +1 weapon
Rune 2: Conscientious. When the bearer of this item contemplates or undertakes a malevolent act, the item enhances pangs of conscience.
Rune 3: Weapon is now a +2
Rune 4: When you hit an Undead, it takes an additional 1d6 Fire damage
Rune 5: This item was created by the foes of Undead. If the culture or creatures are still around, they might recognize the item and single out the bearer as an enemy.
Rune 6: Now a +3
Rune 7: Sentinel. This item glows faintly when Undead are within 120 feet of it.
So, your weapon is +3, glows when Undead are near, is recognizable by Undead, and deals an extra 1d6 of fire damage to Undead. It is made of Mythril, and can communicate with you, effectively acting as your brain and speaking for you.
As for the weapon's type...I leave that up to you, but it must be melee, and does not have the Thrown Property. Once you choose, it cannot be changed. A blade of some kind is thematic, but if you prefer something else, then it won't kill the game. Whatever makes you feel happy. Also, it is sent to Fulgrim, as part of his quest. No one else may attune to it. If Fulgrim were to die, the weapon would fade and disappear.
Note: This is an attempt to give an in game way for Fulgrim to function with an intelligence of 4. He now has a Knight as his companion, and a sword that can translate his thoughts into something more intelligent so Fulgrim doesn't have to speak if he doesn't want to. I picture this as Fulgrim still being in charge, his mind has just been ravaged a bit, and the sword allows him to communicate and the like. That's my thoughts.