When you critically strike with a single-target melee attack that generates combo points, you gain an additional combo point on your target.
Seal Fate is a passive rogue ability learned at level 30 for those with the Assassination specialization. It allows critical strikes from all abilities that add combo points a chance to add another combo point.
Patch changes
Patch 5.1.0 (27-Nov-2012): Seal Fate will now only be activated by melee combo point generating abilities.
Patch 1.3.0 (07-Mar-2005): Fixed bug where Seal Fate was giving two combo points instead of one.