Anduril disappeared from Eren's hands after a while.
His Dagger Arts weapon comprehension kicked in when he held Diceros' set in his hands. In the next moment, a series of piercing and sharp flying weapons attacks wreaked havoc inside the isolation barrier.
Some flying weapon attacks were concentrated at an illusory point of mass, giving them exceptional piercing potential. The other types of attacks released by the Diceros set were so brutal and deadly-- they could butcher a bunch of Adept rankers' bodies in seconds, turning them into minced meat.
Eren let his mind wander off to wherever it wanted to be. He operated on autopilot while handling his weapons. After such a long time handling them, he developed core muscle memories. There were some crucial reflexes that were unique to him. As such, he could think of various topics while practicing weapon comprehension.