Place the same letter 16 times in the letter string below to make a ten-word phrase.

R I R A R A O F R A R L S R K D O F S W T W T S A W D

Answer:

The missing letter is E: 

EERIE AREA OF RARE EELS REEKED OF SWEET WET SEAWEED

Credit:
Joseph Appel  - Nod to Will Shortz