Nilidon was an elven druid appearing in The World Tree, the fifth part of Heroes Chronicles.
Tarnum the Immortal Hero was trying to find the World Tree, the source of all life, so that he could protect it from the mad god Vorr. Tarnum had found some secret caves in a remote mountain, and had come across some caverns full of trees, flowers and animals, and he guessed it was connected to the World Tree, but hadn't found the Tree itself. One day, the druid Nilidon arrived to aid him against Vorr. The rest of his army didn't trust the elf at first, as elves and barbarians have never been friends, but Tarnum greeted him warmly.
Nilidon told him that the elves had once been the guardians of the World Tree, but they had nearly failed in their task, and the barbarian Ancestors had taken over as protectors. Tarnum asked him where the World Tree was, and Nilidon looked at him strangely, explaining that they were actually IN the World Tree, and that the lush tunnels around him were like the veins of the Tree. Tarnum had been fooled by a translation error - what the barbarians called "the World Tree" was known to the elves as "the Roots of Life". While Tarnum had been fighting the necromancers and barbarians following Vorr, his enemy had been spreading a blight on all the plants and scorching the rich soil, destroying the Tree from within.
When Tarnum and his men came across a small waterfall, they were about to drink their fill, but they were saved by Nilidon's druid skills. The elf sensed that something was wrong and warned them that the water was poisoned. They later found a chest sunk in the shallow water, and realized that their enemy had tried to kill them.
Tarnum later managed to convince King Targor, leader of the Followers of Vorr, to abandon the mad god, and when the god fled, the barbarians could defeat the evil necromancers. It's unknown what happened to Nilidon after this, as he didn't follow Tarnum on his hunt for Vorr.