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.
Seeking a spell for personal reasons, the Seer asked Julian Turner to help her steal the grimoire, stating that it likely also contained a spell to break the connection between Morrigan and Wyatt. They succeeded in stealing the grimoire, though they were forced to return in when Morrigan threatened to kill innocents at P3. However, Talia was able to use her magic to copy the spell before returning it.