Coeddil

Coeddil is one of the three Treeman Elders remaining within the realms of Athel Loren. It was Coeddil who, at the Battle of Anguish, scattered Morghur the Corruptor’s forces and seized the beast himself. As Morghur attempted to free himself, Ariel smote the creature. 21

But Coeddil’s distrustful attitude of the Wood Elves is legend. He is so incredibly ancient that it is difficult to ascertain his motivation, for Coeddil has forgotten more than many younger beings — the Elves included — will ever know. 57

History
When the worldroots had been breached by the Elves, long ago, it was Coeddil who had for long centuries directed the great forest’s wrath against its despoilers, consumed with bitter fury and demanded that the interlopers be slain.

At first, Adanhu, oldest and wisest of Elders, agreed with Coeddil, but eventually relented. Eldest he was and fearful of change, but so too did he fear that the forest’s survival might have come only by sacrificing the gender and peaceable side of their nature. Unchecked, the spirits of the great forest would become an enemy as dire as that which they had fought, and Durthu’s bargain presented some small hope that such a destiny could be prevented. Though Coeddil railed angrily against the decision, Adanhu agreed that Astarielle’s heirs could remain within the great forest until their homeland was safe once more — provided that they never awakened from their magical sleep whilst within its bounds. 17

During the Season of Rebirth, in the wake of victory, the Elves began to dwell within the borders of Athel Loren, for they feared reprisals from the Dwarfs more than they did the capricious will of the forest. The forest did not resist, and the Elves wondered why. The wayward children of Ulthuan never heard the harsh words that raged between Adanhu and Coeddil as they argued over their fate. Durthu, instigator of the current situation, remained silent through it all. He had suffered greatly from the Dwarfen axes; his once-kind nature had all but fallen away, and he no longer fully trusted his own judgement. Ultimately, Adanhu’s will prevailed, and the forest now opened up many of its secrets to the Elves, though the Dryads and Treemen took care never to intentionally reveal themselves. 18

Five hundred years after the Battle of Anguish, Athel Loren once more knew internal strife. Coeddil, driven perhaps by a last taint of Morghur’s madness, and who had begun to harbour a deep resentment of the Elves, sought to disrupt Orion’s rebirth. That winter, Coeddil and his Dryad handmaidens did not sleep, but bided until Ariel began her own long slumbers within the Oak of Ages. With much of the forest quiescent, and the Elves unaware of his intent, the ancient strode to King’s Glade and slaughtered all he could find, for if no Wild Riders survived to lead the ritual of rebirth, Orion would be severely weakened — if indeed he could be summoned at all.

Ariel was abruptly awakened from her slumbers as the first Elven blood was spilt. In a great rage she sped to where the Wild Riders fought for their lives. Against Ariel’s fury Coeddil and his followers could not endure. Summoning all her awesome power, the Mage Queen scattered the ancient’s handmaidens and cast down the tree lord. Though Ariel dearly wished to slay the spirits for the damage they had caused and the blood they had shed, Ariel could no more end their existence than sever a portion of her own soul, for Coeddil was still bound to Athel Loren, and Ariel was bound to the forest. Instead she imprisoned the Elder, and the Dryads who had followed him, in the Wildwood — the dark southwestern corner of Athel Loren where no Elves dwelt. The Wildwood was then encircled with waystones, and Coeddil was abandoned amongst the shadow-glades to brood upon his betrayal.

Since that day, no Elf has set foot in Coeddil’s prison, for to do so is to walk with death as their only companion. Coeddil may silently contemplate his fate, but his handmaidens have been driven mad by their exile, and restlessly stalk the glades with cruel desires in their hearts. 21

Drycha's Onslaught
In the year 19 VI, Drycha, Handmaiden of Coeddil, gathered to her a great warweald of forest spirits, and rampaged through southern Bretonnia. After defeating the Duke of Carcassonne at the crossings of the Brienne, she assailed the walls of La Chald Abbey. By the time Bretonnian reinforcements arrived, the abbey had been tom down and its relics stolen. pg.28  In 511 VI, after a seemingly senseless campaign of slaughter in the lands surrounding Parravon, Drycha finally acquired the last relic she had been searching for. Stealing into Athel Loren during the height of winter, the Branchwraith unleashed the power of her stolen artefacts, and shattered the wards Ariel had placed to bind Coeddil into the Wildwood. Alerted too late, Rangers of the Wildwood rushed to Coeddil's prison, only to find the Elder gone. 33

Source
Warhammer Armies: Wood Elves (8th Edition) -- pg. 17 18 21 28 33 57