Saundor

Saundor was an archfey living in the Shademirk Bog in the Feywild and the former owner of Fenthras, Wrath of the Fey Warden. Betrayed and forgotten by his lover, Saundor was left to his own despair, corrupting the Shademirk Bog. Residing inside an enormous tree in the middle of the bog, Saundor longed for someone to heal his wounds and cleanse his heart:  a companion, a friend. As an NPC, he was played by Matthew Mercer.

Description
Saundor was a thin, elf-like individual covered in a mass of plants, leaves, and thorns. The only flesh-like portion of his body was his face. His eyes glowed bright yellow. He could connect his body to the tree he resided in through tendrils of plant matter that attached to his back and shoulders.

Encounter with Vox Machina
While in the Shademirk Bog, as part of Vox Machina's preparations to enter the cancerous tree rumored to contain Fenthras, Keyleth cast Commune with Nature to learn about the tree. Asking the environment what was fueling the tree, she heard the name "Saundor" for the first time.

The next day, the group approached the tree, with Scanlan Shorthalt announcing the group's presence to Saundor under the pretense of seeking his blessing as his devotees. Keyleth cast Speak with Plants, asking if Saundor could hear her, but she met with silence. She then announced herself as an Ashari druid and asked if he was present, and an ominous voice replied in her head, "I'm here." Keyleth responded, "We would like to see your heart." The vines and bark opened, and a small, dark passage into the tree opened before her, and the group entered.

Despite his initial interest in Vex'ahlia, Saundor became angry at her rejection and assault. Eventually, he succeeded in knocking her unconscious. When Scanlan's Healing Word returned her to consciousness, Saundor used a legendary action to attack her again and cast a spell at her.

Frustrated by Saundor's mobility keeping him out of melee range, Grog casually walked in front of Vex'ahlia to shield her with his own body. The goliath turned toward Saundor and admonished, "I've broken some hearts in my time, but you, stupid Romeo, are takin' it way too far—guess what!  Nobody cares!"  At that point, Grog hurled his Javelin of Lightning at Saundor, impaling the archfey through the jaw. After the physical impact, the javelin's lightning burst outward, blasting Saundor's upper body into pieces throughout the room.

After Saundor's death, the black ichor that seeped out of the plants of the Shademirk Bog ceased flowing and started hardening like amber.

Abilities
Saundor exerted control over his natural environment as an extension of his consciousness, covering himself in natural armor and commanding the nearby vines and treants, as well as ejecting arrow-like, viney spikes from his hand as ammunition for his bow, Fenthras, Wrath of the Fey Warden. He was able to stride through the tree that constituted his lair. Saundor was able to regenerate during combat. Additionally, he possessed the following abilities:
 * Lair Actions:
 * [unknown]:  A gas cloud covering an approximately 10-foot radius, requiring a Constitution saving throw.
 * [unknown]:  A gas cloud covering an approximately 20-foot radius.  While Cloudkill and Stinking Cloud require a Constitution saving throw, and Incendiary Cloud requires a Dexterity saving throw, this spell required a Wisdom saving throw.
 * [unknown]:  Vines from the nearby walls of the tree whipped out and grabbed nearby targets.
 * Legendary Actions:
 * [unknown bow attack]
 * [unknown spell]:  Required a Dexterity saving throw; costed two legendary actions; successfully prevented by a 5th-level Counterspell.
 * Blight
 * Reverse Gravity
 * Legendary Resistance:  Allowed Saundor to automatically succeed on a failed saving throw.
 * Special Attacks:
 * [unknown attack]:  Saundor's fingers curled up into "giant, extremely hard, petrified root clusters".
 * Brambleshot Attack:  Saundor shot viney arrows that erupted upon contact as thorny vines, wrapping around their target.  This attack was likely the Bramble Shot from Fenthras.
 * Spells:
 * Blight

Trivia

 * Matthew Mercer's character sheet for Saundor was briefly shown in "Behind the Screen", the ninth episode of GM Tips with Matt Mercer, with his name spelled as "Sondur".  The character sheet was written before, and after that episode, Matt stated twice on Twitter that the correct spelling is "Saundor".   This wiki uses the more recent "Saundor" spelling.
 * Travis Willingham speculated that the tree used to be a person whose heart was broken, which was not far from the truth.