He actually had a bit more lore that ties into Xavius, the nightmare and Nzoth. because many people never read the Stormrage book a chunk of his lore was lost to them.
https://wow.gamepedia.com/Fandral_St...rion_Stormrage
At some point in the months following the Lich King's defeat, Fandral gathered many druids to Teldrassil, unfolding a master plan to heal the giant tree. Naralex, Broll Bearmantle and Hamuul Runetotem were among those he called. He intended to use the Idol of Remulos to achieve this, against Bearmantle's better judgment. Before the ritual could be completed, however, Tyrande arrived, having been warned by Elune that Malfurion's life was in greater danger than before. It was discovered that rather than purifying the tree, Fandral was, in fact, responsible for its being corrupted in the first place. He was the one responsible for grafting a branch of the Nightmare Lord onto Teldrassil and he was the one who led the Cenarion Circle in feeding their powers to the World Tree, thus feeding its corruption. Fandral however, was no longer in control of his own actions. Rather, he was an unwitting pawn and puppet[23] of the father of all satyrs, Xavius, who had been transformed into a tree by Malfurion during the War of the Ancients.
Xavius, in his guise as the Nightmare Lord, servant of the entities behind the Emerald Nightmare's encroachment on the dream, had appeared to Fandral in the form of his son Valstann. Fandral had never been able to reconcile his failure to save his son, and the visitations played on his guilt and rage, eventually driving him to a kind of madness that blinded him to the truth. He chose to believe that Valstann had returned and that all of his actions at "Valstann's" urging were for the greater good. Using [Morrowgrain] to keep Malfurion trapped in the dream, grafting a branch of Xavius' nightmare form onto the young Teldrassil, anything and everything Fandral did he did believing it would fully restore his son to life.
Though Naralex, Shandris Feathermoon, and Hamuul Runetotem had discovered Fandral´s betrayal, Fandral silenced the three and had them imprisoned in the Cenarion Enclave. Only after Malfurion returned to free them was the truth finally revealed.
During the War Against the Nightmare Fandral's delusion ended up being forcibly shattered, as the image of Valstann was destroyed and he fell into a state of near-catatonia. Malfurion managed to defeat Xavius, but Fandral was left a broken husk, forced to endure the loss of his beloved son for a second time. After the Nightmare's defeat, the druids brought Staghelm to Illidan Stormrage's former prison in Mount Hyjal in the hope that he would recover, though many suspected that his mind was permanently broken.[24]