Marlin originates as a foot soldier in an unknown army. Escaping across a desert with his companion Adda, he finds a Holy Grail filled with water. After Adda dies trying to drink from it, Marlin drinks and gains immortality and magical abilities. His magic unintentionally transforms the desert into grasslands.

Centuries later, Marlin rises as a renowned healer. He takes a young apprentice, gifting him a wand. Marlin creates a magical box containing a hat designed to absorb and accumulate magic, protected by an enchantment allowing only those free of darkness to open it.

He encounters Nimue, who seeks vengeance against Vortigan for destroying her village. They fall in love. Marlin reveals his immortality, fearing he will outlive Nimue. To relinquish his powers, he plans to reforge the Holy Grail into Excalibur using humanity's original fire. Unbeknownst to him, Nimue drinks from the Grail, gaining her own immortality and magic. During the forging, Vortigan attacks. Nimue kills him by ripping out his heart, an act corrupting her with darkness and turning her into the first Dark One. She breaks Excalibur and steals the last spark of the original fire.

Marlin fashions the broken sword tip into a dagger, tethering it to Nimue to control her as the Dark One. The dagger compels obedience from its possessor. He places the broken sword in a stone. Later, he attempts to kill Nimue with the dagger but cannot due to lingering love. Nimue captures one of his tears on the dagger, transforming him into a tree. Despite this, he retains the ability to communicate prophecies through his apprentice.

Trapped for centuries, Marlin prophesies a boy named Arthur will become king by pulling Excalibur from the stone. Arthur fulfills this but hides the sword's broken state. Marlin also appears as an usher to a young Emma, warning her never to pull the sword from the stone.

After Emma becomes the Dark One, Marlin's apprentice directs her family to find him. In Camelot, King Arthur reveals Marlin is trapped in a tree. Using a tear of lost love from Henry, Emma and Regina free him. Marlin aids them by freeing prisoners and reversing enchantments. When Hook is fatally wounded, Emma uses the darkness within Marlin to tether Hook to Excalibur, saving his life but turning him into a Dark One. Seeking revenge, Hook murders Marlin to cast a Dark Curse. His body is later destroyed by Emma.

Following his death, a message from Marlin appears via the Crimson Crown, indicating his influence persists. His backstory reveals Nimue was his ex-fiancée. He operates with a broader perspective, championing balance above personal desires. Marlin views morality as fluid, with actions driven by conviction rather than absolute good or evil. His guidance centers on emotional readiness and the consequences of wielding power.