Morrigan's Grimoire is an ancient and powerful tome of magic owned by the Morrigan. It contains all her spells and potions.
The Morrigan presumably used a potion from her grimoire to tie her essence to the soul of King Arthur. When he was reborn as Wyatt Halliwell centuries later, the Morrigan was resurrected when he reached the same age as Arthur. Morrigan then used a spell to summon her book through time to the present.