The Gean Canach is an Old Religion magical creature similar to an enormous slug, fearsome for those who have magic. Forged by the tears of the Earth Mother Nemaine, it devours the magic of others, draining them of their power.
HistoryAlthough such magical creatures were believed to have been entirely wiped out during the Great Purge as all the creatures of the Old Religion, Morgana obtained a box, bearing the symbol of the Gean Canach on it, that contained the infamous slug.
Morgana considered the creature her weapon to win the war against her brother Arthur as it was her belief that "Arthur is nothing without Emrys and Emrys is nothing without magic". She tested the Gean Canach power by stripping an ally of hers, the sorcerer Ari, of his magic.
One of her Saxon warriors infiltrated Camelot and put the box with the creature under Merlin's bed. When he came home from the tavern, Merlin was attacked by the slug and, before Gaius could kill it with a shovel, the creature consumed the young warlock's powers, leaving him with no magic (The Diamond of the Day: Part 1).
Merlin travelled to the birthplace of magic itself, the Crystal Cave, to discover how to restore his powers. Being magic itself personified, he was able to regenerate his powers (The Diamond of the Day: Part 2).
The Gean Canach resembles a big slug and it seems to reside in a box that bears a mark of the Old Religion. It can be killed like a mortal creature, as Gaius used a shovel as weapon.
Its name comes from the Old Religion to which the creature belongs.
The ability of the magical creature is to strip a magic user of their power by sucking them out of the person. It is unknown if a victim of the Gean Canach permanently lost their powers or if their magic could return with time although, judging by the fear shown by Ari, it could be assumed that there was no way to restore the powers after the attack of the creature.
The spell Morgana casts ("Aweax þu meteþearfende! Þicge þu þone drycræft þe þinan deorcan mode gefylþ") might be necessary to wake the creature or convey it an order, as the Gean Canach crawls outisde the box to attack Merlin without any prompting.