Merlin is a legendary figure best known as the wizard featured in Arthurian legend.
At some point he and the devil supposedly created Stonehenge. He was once the court magician to King Arthur. One day, he met Nimue, who seduced him and learned all of his secrets, after which she buried him alive.
As Gruagach ran through the forest, after being banished by Nimue, he tripped on the lid of Merlin's grave and was greeted by the spirit of the wizard. After realizing who he was, Gruagach begged for mercy, but Merlin said that he would receive none. Merlin then told Nimue's story, and showed Gruagach the consequences of setting her free, including her murder of Queen Mab which doomed Gruagach's entire race to extinction. Merlin then told Gruagach that he was cursed to live to see the outcome of this war and that he would not die "till the world ceases to turn."
Gruagach was later found by Morgan Le Fey, Merlin's one-time apprentice, who took pity on Gruagach and broke Merlin's curse of undeath on him, and offered to lead him to "the next world" and save him from witnessing the outcome of the battle between Nimue and Hellboy, which she had no desire to see either. As she led him away, they ignored Merlin's howl of protest from his grave.