With the black onyx as core, you target one or more medium or larger corpses within range. The corpses weave together and combine into one form around the onyx.   The Ascended Undead uses the statistics of a CR 9 or lower Undead of your choice. It's Intelligence, Wisdom, and Charisma scores are increased by amount equal to the number of corpses consumed in its creation, to a maximum score of 20. The Undead disappears when it drops to 0 hit points or when the spell ends.   The creature is friendly to you and your companions. It acts on each of your turns. You decide what action it takes and how it moves. The DM has the creature's statistics and resolves all of its actions and movement.   If the spell lasts for the full duration, the creature no longer disappears and you no longer control the creature. It might remain friendly to you, depending on how you have treated it.