After securing the Mandrake Formation and its accompanying compendium, Eren expressed his gratitude once more to Minerva before proceeding to scrutinize the remaining rewards.
Next on his inventory was a transcendent-grade potion enabling the casting of elemental spells with the powers of an S-Rank entity. This consumable, a one-time-use elixir, granted Eren temporary access to a singular S-Rank spell.
There were three potion bottles of this kind. This means Eren could technically cast three S-Rank elemental spells. There would be side effects but Minerva assured that they wouldn't be permanent or life threatening.