It's a branch/off follow up story. I don't want to spoil too much, but it has to do with the Next Big Prophecy at the end of the last book in the original Percy Jackson Series.
NO NICO D'ANGELO! and the gods are being bitches as usual! but what was worst is that the prophesy rachel predicted, occurred too soon IMO rick riordan should at least had a time skip! But was humorous as always and nice that it was a long story