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If the spirit has the Possession Charm (detailed in Mage 20, under Umbrood Spirit Entities), then that entity can try to take over the character and use its Charms to influence that host. The systems involved are detailed in Mage 20, under the Possession entry on pp. 493-494. Unless the Storyteller decides otherwise, that entity cannot use its Charms through the human character… and, more importantly, the character cannot access the Charms. A Storyteller may make exceptions in certain limited cases (like a wind-demon summoning storms through a possessed human character), but in general we recommend against it.