The Gean Canach, a name which comes from the language of the Old Religion, is a magical creature similar to an enormous slug, fearsome for those who have magic. It was forged by the tears of the Earth Mother Nemaine and it devours the magic of others, draining them of their power.
Although such magical animals were believed to have been entirely wiped out during the Great Purge, 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).
Merlino, being magic itself personified, eventually regenerated his powers (The Diamond of the Day: Part 2).
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 crawl outisde the box to attack Merlin without any prompting.