You cast a curse targeting two individuals. Both targets must make a Charisma saving throw (which they can choose to fail). If both targets fail their saving throws, for the duration of the spell their souls are swapped. A soul controls the body it inhabits. It gains all ability scores and actions the body had (besides legendary actions or legendary resistance), but retains its own spellcasting (if it has the spellcasting, innate or otherwise), and has disadvantage on all attack rolls and strength, dexterity, and constitution saving throws for 1d4 turns after swapping bodies.
At the end of a swapped creatures turn, it can choose to repeat the saving throw, ending the effect on a successful save. If its CR (or character level if it has no CR) is higher than the body of the creature it is in, it has advantage on the save.
If a creature dies while while its soul is swapped, the souls return to their original bodies. If a soul was in a dying creature that returns to a living body, that creature takes 5d10 necrotic damage.