I might be trying this sometime soon in a Bleach/World of Darkness/Books of Magic inspired PbtA thingy. Teenage exorcists and psychopomps in a necropunk action drama.
Each player character has 2 “bonds”. These are statements of something they can’t, must, or won’t do. They also have six tick boxes to track “blood”. However, only 3 are available at the campaign start.
When you spend blood, fill in a box. You can spend blood to cheat death, get a 12+ result on a roll, or activate certain powers.
When you do, also mark XP and Gaze into the Ether. Each playbook has a different cost on a 6 or less.
At the end of each session, ask:
1. Have I found a new bond to replace an old one? This could be a complete change or a gradual evolution. If so, write a new bond in its place, mark XP, and regain a spent blood mark.
2. Have either of my bonds driven me to an extreme? If so, mark XP and gain access to one more blood box (6 max). [I have no idea yet, for what to do after the maximum is reached]
If you ever spend all 6 blood and at the end of the session can’t answer “Yes” to question 1 & regain blood, you become a hollow spirit, a poltergeist or grudge, forever transformed by your obsessions. Game over.
/sub