Two target players exchange life totals. You create an X/X colorless Horror artifact creature token, where X is the difference between those players' life totals.
When life totals are exchanged, each player gains or loses the amount of life necessary to equal the other player’s life total. For example, if one player has 10 life and the other has 17 life, the first player gains 7 life and the other one loses 7 life. Replacement effects may modify these gains and losses, and triggered abilities may trigger on them.
If a player can’t gain life, that player can’t exchange life totals with a player with a higher life total. Similarly, a player who can’t lose life can’t exchange life totals with a player with a lower life total.
Use the difference between the life totals after the exchange to determine the size of the Horror token. Because replacement effects can modify the life gain and life loss, this number may be different from what it was before the exchange.