Woodsung weapons and armor are crafted from elder trees via song by archdruids and fey to serve a specific purpose, what the purpose is is up to the dm of player. 

Woodsung weapons grant proficiency to their weapon type, when attuned.

When you roll a natural 20, after damage is rolled,you may cut off a limb of a creature that would not kill it; such as an arm or a leg. If the damage would inherently kill the creature you cleave or sunder the creature instead. 

Regardless of the option, a creature killed or injured in this fashion has flora magically sprout from the now lifeless body part or corpse.

The extent, speed, and flora may be of your choosing but cannot exceed the size of the body part or corpse. You cannot make a huge  300 foot tree grow out of a severed hand in one action, BRIAN. No one should have to have said that, but here we are, BRIAN.

Flora adorns the greatsword, as it is made of wood. Describe the sword in detail in the notes section and utilize that description for the features in which flora appear. You cannot animate any part of the greatswords's Flora, nor use them for spell components, BRIAN. 

 

Flora facade:

once per long rest:you may, as a reaction, stab the greatsword into a patch of solid earth(rock, dirt, soil, ect) that is neither moving or on fire. When you do so your movement speed becomes 0, and you gain 10 A.C. and +5 to saving throws as the flora in the sword surges suddenly to grow around you absorbing some of the damage before wilting and withering away. If the damage is fire, magical or non magical, you lose your entire next turn in combat. You cannot be moving either, BRIAN, using this feature falling down a a cliff will not save you for an example.

Flora facade may be used out of combat, to your dm's discretion. You cannot use it to make a shelter, but you can use it to hide for 1 round(6 seconds,BRIAN).

You gain vulnerability to fire damage, while attuned to this weapon, even if it is not on your person. Handing the weapon to someone will not give them this vulnerability, BRIAN.

while attuned:setting the weapon aflame on purpose, after the dm asks you "are you absolutely tippy top of the box positive that you have the inherent desire to do this and suffer the consequences?" destroys the Woodsung greatsword, and the attuned dies .in the fire. RIP BRIAN.

You have a +3 bonus to attack and damage rolls made with this magic weapon, or something.

Proficiency with a Greatsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Vulnerability: Fire, Proficiency: Greatsword, Flora facade: +10 ac, +5 on saving throws, fire damage means you lose your entire next turn., Meditation in a tree, Damage, Combat, Heavy, Two-Handed, Graze