An apex predator's Individuality.
Garos instinctively felt that Eren had cultivated a kind of Individuality for himself within merely 30 years which he was unable to do even after living for around 2000 years.
The more he fought Eren, the more he needed to assure himself that he was still better. This feeling creeped up on him sneakily, making him want to dominate Eren at any cost.
The King's Hand subconsciously felt that only by showing off his strength as a Sage could he feel better about himself.
"Enough. No matter how many tricks you have up your sleeves, you are still weak!"
Garos exclaimed as he deployed his water-element spell. In the next moment, a huge mass of water appeared from thin air and surrounded Eren from all angles.