You are irrevocably destined for an unhappy end. Whether you will meet with tragic failure of epic proportions, an unheroic and embarrassing death, a quiet and painful one, or maybe an unjust one is up to the GM, and you may or may not know your fate. Perhaps you do know and you struggle nobly to avoid your fate ala Oedipus, each step only bringing your disgrace closer. Perhaps your confidence is shattered by the doom shrouding you, resulting in a self-imposed curse of purposeful failures. Maybe you meet your fate and as a result, come back as a ghost to haunt a character that seemingly abandoned you.
Essentially, in game terms, this gives the GM license to plan out in secret the fate of the character. A good story tool to introduce this Fault is the wandering mystic who reads the character's fortune and asks if she really wants to know what happens in the end. From them on, the GM should make a point to ensure the character becomes a tragic figure and meets a fitting and unnerving end, perhaps greatly affecting the other characters in the story. The GM can also tailor events and outcomes to suit a Bleak Destiny and is the final and absolute arbiter in any of the character's goals. This should not give the GM free reign to take out aggression on the character or torment him/her for amusement. Bleak Destiny ties in nicely with many other Supernatural Faults, such as Cursed and Haunted, but even the most heroic and noble of the Called have found themselves dying in shame and agony — that, after all, is the stuff tragedy is made of.