By reading Eris's notes on sword logic and the secrets of the Hive, you see she believes she can claim deeper power over the Throne World by continuing Savathûn's experiments with the Light.

Attuning to an element will allow you to explore Savathûn's Spire more efficiently and augment your own capabilities.

Choose an element and begin the ritual.

(Your decision will impact your initial rewards, though you will ultimately be able to attune with all elements.)