Gittin 72 - Conditional Divorce
If a husband told his wife, "This is your Get if I die," or, "This is your Get if I die from this illness," or "This is your Get after my death," he has not said anything. In all three cases, the Get is void because the husband stipulated that it will take effect after his death and this is impossible.
If he said, "This is your Get from today if I die from this illness," - it is a valid Get. It takes effect retroactively, but care must be taken not to invalidate the Get by living together.