In volume 14, we're introduced to Farnese and her Knights of the Holy Iron Chain just as they find the prophesized Red Lake that marks the advent of the Falcon of Darkness. In Japanese, the words used for "red lake" are 赤き湖, pronounced あかき みずうみ (AKAKI MIZUUMI).
赤 (AKA) means red, and if you remember our entry about the Black Swordsman, you'll recall that adding an "i" at the end turns the word into an adjective. Like 黒い (KUROI) or 赤い (AKAI). So why is it 赤き (AKAKI) in this case? Because it's an archaic form that is mostly used to sound poetic nowadays. It's not an adjective either but the attributive (adnominal) base of the archaic adjective 赤し. Puella had to explain that to me just now so don't focus on it too much.
Anyway, here it serves to convey that they're talking of an ancient prophecy from the days of yore.
As for 湖 (MIZUUMI), it's the standard word for lake. That pronunciation comes from the words 水 (MIZU), meaning water, and 海 (UMI), which strictly means "sea" nowadays but used to refer to any large body of water in ancient times. When the Chinese writing system was adopted by Japan, the pronunciation MIZUUMI was given to the Chinese character for lake: 湖.