I assume that you are referring to the "Defensive Roll" feat that lets you make a reflex save when you are about to drop to 0 hp in order to try to halve the damage you take from the attack:
Defensive Roll - NWN2Wiki, the Neverwinter Nights 2 wiki - Races, classes, skills, and more
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One way of doing this is to make it an Encounter or Daily power, something like this:
Defensive Roll - Paragon Tier Feat
Prerequisites: Dex 17
Effects: You have access to the following power:
Defensive Roll - Feat Power
Daily* - Immediate Interrupt
Trigger: An attack hits you that would put you at 0 or fewer hit points.
Effect: Take half damage from the triggering attack. Ongoing damage the triggering attack deals is not affected.
*You could change this to an Encounter power if you wanted it to be more powerful.