The Holy Tree Flagoon (Japanese: 神聖樹 Kanji translation: Holy Tree, Romaji: furagūn) is a massive tree growing in the center of Sairaag. The tree is so large that many people mistake it for a forest on their first visit.
The tree was planted by the Swordsman of Light after she killed the Demon Beast Zanaffar that destroyed the first Sairaag. A flood of infinite miasma poured out of the slain Zanaffar's body, creating the Miasma Forest. The Swordsman planted the Holy Tree to absorb and purify the miasma.
When Copy Rezo arrived in Sairaag, Flagoon began to feed on his miasma. Being a half-demon, Copy Rezo generated so much miasma that the tree began to visibly grow. Copy Rezo attempted to destroy Flagoon, which proved to be his undoing; Sylphiel Nels Lahda cast Recovery on the tree, and in the process of regrowing it completely consumed the demon portion of his body.
When Hellmaster Fibrizo came to Sairaag, Flagoon attempted to absorb his miasma. However, Hellmaster's miasma was too much and/or too potent, and the tree died in the process. Fibrizo decided to build his temple in its place, in order to neutralize the miasma surrounding it and the Miasma Forest.
The Blessed Blade is a magical sword that was discovered within Flagoon, and had a connection with the tree.
A tree of the same kind as Flagoon is planted by Pokota around the outskirts of Saillune City to absorb the miasma of the recently destroyed Zanaffar at the end of Slayers REVOLUTION.