The Nature Guardian is a boss mob that has the appearance of a deformed tree holding a scythe. It is one of the many elemental guardians and is designated to be encountered in the late game. However, due to its unique summoning condition, the Nature Guardian can be encountered relatively early.
Arcane Compendium Entry
"The Nature Guardian is a fearsome opponent. Boasting a deadly scythe and the skill to use it, only the most powerful or foolish would seek it out. The few who have survived an encounter state that it is just as dangerous at range as in melee, if not more."
Unlike the other elemental guardians, the summoning ritual for the Nature Guardian does not require the use of inlays with a sacrificial item in the end. Instead, it appears when the player kills several Dryads, a mob that appears as a plant-like humanoid. Without prior knowledge of this, the player could have an unexpected encounter with the Nature Guardian.
The summoning appears to require 5 Dryads be killed in less than 400 ticks (20 seconds) each. The Nature Guardian will spawn at the location that the last Dryad is killed, regardless of whether every Dryad was killed by the same player or not. It should be noted that any Dryads killed by a Nature Guardian will not count towards summoning another one but, besides that, you could theoretically summon many, provided enough Dryads.
The Nature Guardian is a very resilient opponent, due to it having 500 health points, which is equivalent to 250 hearts. It has powerful damage resistance reducing many attacks to 1 damage. It also wields a deadly scythe that can quickly deal heavy melee and ranged damage.
When the player is in melee range, the Nature Guardian will swing its scythe or perform a spin attack that deals several blows in quick succession. For its ranged attack, the guardian will throw its scythe at the player. This can instantly kill unarmored players and deal significant damage to even those who are well armored.
It's 4-5 blocks tall so you can simply spawn it in a two high room and wait for it to suffocate. Other options include making a ceiling of lava for it, so long as it isn't allowed to swim up. While stuck, the player will have a very difficult time hitting it, often being impossible to hit with projectiles or swords though it may be possible with AoE spells. However, with just suffocation or lava damage, it takes a long time to take down 500 health. Similarly, you can summon it in a room such that the space it is in is 4 or 5 high and the rest of the room is shorter, making it so it can't move and still allowing you to hit it. This also allows you to run away if things get rough as its ranged attacks seem to only reach about 10 blocks or so.
It is recommended that the player be at least a magic level of 60 or higher before taking on the Nature Guardian.
The weak spot of the Nature Guardian seems to be the wooden block surrounded by the swirling yellow things that may be considered it's "roots."
A rare drop from the guardian is the Nature Guardian's Scythe. When the player uses the weapon, it will launch a scythe that will damage enemies and destroy certain blocks in its path for a cost of 250 mana.