A creature that ingests this poison suffers no effect until the stroke of midnight. They also do not know that they have been poisoned.
If the poison has not been neutralized before then, the creature must succeed on the defensive roll.
How this works is the GM rolls the DC17 fortitude in secret. Usually over the long rest the damage is applied.
You inspect the vial, its contents shimmering like obsidian tears. The liquid holds a hidden menace, dormant until midnight strikes. Unbeknownst to the victim, it waits to unleash its insidious effect, sealed fate decided by a silent, secret roll.