NMAC IN NIGHT OP DURING VISUAL APCH PROC.

Narrative

WE WERE INBOUND TO LUK ON AN IFR FLT PLAN WITH GOOD VMC AND ARPT IN SIGHT. THE APCH CTLR CLRED US FOR A VISUAL APCH TO RWY 25 WITH INSTRUCTIONS TO KEEP OUR 'SPD UP.' WE WERE POSITIONED ON A WIDE L BASE TO RWY 25 WHEN TRACON HANDED US OFF TO LUK TWR. WHEN WE RPTED OUR POS TO THE TWR CTLR; HE ADVISED US OF VFR INBOUND TFC (A PISTON TWIN) AT OUR 1 O'CLOCK POS AND INSTRUCTED ME TO WIDEN MY PATTERN TO FOLLOW THAT TFC. I GAINED VISUAL CONTACT WITH THE PISTON ACFT AND TURNED TO A HDG OF APPROX 360 DEGS FOR LATERAL SEPARATION. ON THIS HDG; MY COPLT CALLED TO MY ATTN INBOUND TFC AT OUR 12:30 O'CLOCK POS. HE THEN ASKED THE TWR CTLR IF HE WAS WORKING THE ACFT. THE TWR CTLR RESPONDED THAT THE TFC WAS AN INBOUND CITATION STILL ON APCH CTL FREQ. AT THIS TIME MY ACFT AND THE CITATION WERE ON CONVERGING FLT PATHS. I INSTRUCTED MY COPLT TO MAINTAIN VISUAL CONTACT WITH THE CITATION AS I MANEUVERED MY ACFT IN A L TURN FOR SPACING BEHIND THE INBOUND PISTON ACFT NOW ON FINAL. WHILE IN THE TURN; MY COPLT ADVISED ME THAT THE CITATION TFC WAS ON A COLLISION COURSE WITH US AND THAT I HAD 'BETTER GET IT DOWN.' THERE WAS URGENCY IN HIS ADVISORY; AND IT WAS FOLLOWED BY A TCASII TFC ALERT INDICATING THE CLOSE PROX OF THE CITATION (LESS THAN 400 FT VERT SEPARATION). I NOSED MY ACFT OVER ATTAINING A SINK-RATE OF 1800 FPM BEFORE INITIATING RECOVERY. AT THIS POINT MY ACFT WAS LINED UP ON FINAL. AS I LOOKED OUT MY COCKPIT SIDE WINDOW; I COULD SEE THE CITATION IN A L TURN PASSING DIRECTLY OVERHEAD WITH LESS THAN 500 FT VERT SEPARATION. MY VISUAL APCH WAS COMPLETED WITHOUT FURTHER INCIDENT. IT APPEARS THAT POOR COM BTWN CVG TRACON AND LUK TWR COUPLED WITH THE FAILURE OF THE CITATION CREW TO MAINTAIN ADEQUATE VISUAL SEPARATION FROM MY ACFT LED TO THIS INCIDENT WITH NEAR TRAGIC CONSEQUENCES.