Bloodletting Puzzle

When Bloodletting Puzzle enters the battlefield,
tap target creature you control. It deals one
damage to the target creature and creates a blood
token.
{2}{t}: Sacrifice a blood token. When you do,
create a ritual counter on Bloodletting Puzzle.
Then, if four or more ritual counters are on Bloodletting Puzzle, transform it (into Serana, Mysterious Woman).