The Black Sword Hack is about the end of the world but there is no pacing mechanic to trigger it end.
I guess that you could establish that reaching level 10 trigger the end but let try something that follow the fiction and the character's actions.
Law and Chaos have a Cosmic Usage Die.
- They could both start at d8 but you can also start with a unbalanced world.
- If a cosmic usage die go under d4 or above d12, it trigger the end of the world.
- When both cosmic dice are equal, the Cosmic Balance is in equilibrium.
At the end of an advenutre:
- Test the usage die of the defeated or hindered faction: on a result of 1-2 it cosmic die step down.
- The lost was extra hurtful (the death of a champion, etc): roll with disadvantage
- The lost was minimal: roll with advantage.
- Test the usage die of the triumphant faction: on a maximum result it step up.
- The trimph is exalted or glorious: roll with advantage
- The trimph is minimal: roll with disadvantage
- When your faction's cosmic die become the highest (or step up?), your Doom dice step up by one for the duration of an adventure.
- For characters aligned with Balance this happen when the cosmic dice reach the same value.