The Heroes of Olympus is a pentalogy of fantasy-adventure novels written by American author Rick Riordan. The novels detail a conflict between Greek demigods, Roman demigods, and Gaia (Roman name Terra). In the fourth book of the series, there is also a fight against Tartarus, which, in Greek mythology, was the darkest and deepest point of the Underworld. The series can be read as a standalone volume, but is meant to be read after Percy Jackson & the Olympians. Riordan introduces Roman mythology in the series alongside several new characters, primarily from the Roman Camp Jupiter. The first book of the series, The Lost Hero, was published on October 12, 2010. The final entry in the series, The Blood of Olympus, was published on October 7, 2014. The story is continued in the sequel series The Trials of Apollo. Source: Wikipedia (en)