You attempt to transfer your soul into the form of a nearby undead creature, thereby controlling it but leaving your own body lifeless.
The target makes a Wisdom saving throw against your spell save DC. A creature whose CR is higher than the caster's level makes their saving throw with advantage. On a success the transfer fails and the spell slot ist wasted. On a failure you take complete control of the undead creature. If the undead has a wisdom score of 12 or higher, they can attempt to repeat the saving throw at the end of each of their turns, ending the effect of Bloodstone's Frightful Joining on them on a success.
Once in control of an undead body you retain your intelligence, wisdom and charisma but use the physical attributes (strength, dexterity and constitution) of the undead you are currently controlling (the DM provides these stats). For the duration of the spell you are able to use any physical attacks and abilities that are inherent to the undead's physique but do not gain access to any magical abilities or spellcasting that the undead creature would usually have. When in doubt about certain attacks or attributes, consult with your DM.
While you are controlling the undead's form, your original body is considered unconscious (your hit points are unaffected by this). You can sever the connection and return your spirit to your body by ending your concentration (no action required). Your spirit is also returned to your body if the undead, who is affected by the spell, drops to 0 HP or your concentration is broken.
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