The answer to the crossword for the term 'SOURCE OF ACHILLES' STRENGTH' is STYX, which is a four-letter word. The Styx is a river in Greek mythology that was believed to be the boundary between Earth and the Underworld. In Greek mythology, it was said that ACHILLES was dipped into the Styx by his mother, Thetis, which made him invulnerable except for his heel, where she held him. This is why the phrase 'ACHILLES' heel' is used to refer to a person's weak spot.