True Potential of Spell Integration
The gluttony pearls had their limits as well.
The pearls were selective when it came to spell amplification and spell stacking. Meaning they could only help Eren amplify or stack the elemental spells of a certain element. This element was dependent upon the ranker's elemental nature, aspect, and attainment from the gluttony pearl was made from.
The spell amplification or spell stacking wouldn't be very effective for elemental spells gluttony pearls do not directly support. This was the reason Eren had prioritized casting fire-element spells at this point. That's because the participant from which the crushed gluttony pearl was made was a fire-element ranker.
Spells that were not related to fire elements received a little boost from spell amplification and spell stacking. However, this little bottleneck in the use of gluttony pearls couldn't stop Eren's enemies from feeling shit-scared by what he had cast.