Shadow Clones.
Ever since he had claimed Shallot's Mirror for himself, Eren was keeping at least three clones active every day at all times to maximize his workflow.
His one clone was dedicated to him practicing his potion-making and artifact-creation skills. There was another clone that Eren had maintained to develop a Ranking Technique suitable for his current Rank.
Lastly, the third clone dealt with business and political affairs related to his city and guild. This third clone worked closely with Agatha, lessening her burden by quite a lot.
Eren wanted to have a fourth clone for himself which could help Maya in creating a Spatial Gate for his travel to the otherworlds. However, he found it exponentially difficult to keep his remaining clones functioning.
