Eren felt like he was in his previous timeline, being a berserker and drinking a berserker potion. Yet, he could feel that the boost he had received from the Demon Beast Transformation spell was a tad bit higher than what the Berserker potion allowed.
And this was when he and Argo were just in the Ace rank. When he had only just started using the spell. He couldn't even imagine how fearsome the boost would get as both he and Argo kept ranking up and got compatible with using the spell.
Eren focused on Altashia and observed that she had more control over the spell because of the tamed mana signature that she was giving off. Her stature had also increased a bit. And her pupils had turned vertical as well.
But apart from these not-so-apparent changes, there were no visible beastly features that could be seen on her body. It seemed that she only strengthened her internal organs and boosted her body stats with the spell's somewhat controlled effects.