The Immortals War took place in 452 HE^[1] in Tortall and neighboring countries, and is called so because of the opening to the Divine Realms and many lesser Immortals coming into the Mortal Realms (i.e. Stormwings, Spidrens, Dragons, Griffins, etc.). This is somewhat of a misnomer, however, because the Immortals were merely allies to Scanra, the Copper Isles, and Carthaki Renegades, not the primary antagonists.^[2] The war occured in the fourth book of The Immortals Quartet, The Realms of the Gods. 

1. ↑ http://www.tamora-pierce.com/faq_timeline.html
2. ↑ FT, Chp 4