From the preface:
In the adventure Princes of the Apocalypse, the cosmic threat of Elemental Evil reaches the Forgotten Realms. This supplement provides new options for player characters in that epic campaign. If your Dungeon Master allows, these options can be used in other campaigns as well.
Create Heroic Characters to Conquer the Elements!
Not inherently evil, elemental power can be mastered by those with both malevolent and benign intentions. The Elemental Evil Player’s Companion provides everything players need to build a character that is tied directly into the Elemental Evil storyline.
New race options include the aarakocra, deep gnome, genasi, and goliath. Additionally, a plethora of new spells put the elements directly at your command.
The Elemental Evil Player’s Companion, was original designed by Richard Baker, Robert J. Schwalb and Stephen Schubert, with additional design and development by Wizards D&D R&D.
This accessory is specifically meant to support the Elemental Evil–Princes of the Apocalypse adventure product.