It was only after throwing himself into the dark pit that opened that Uriel realized that the trapdoor will soon close by itself, meaning it was a one way trip and the only way out was through. Being trapped in a sealed chamber was already bad, but for Uriel it could possibly mean an eternity of that as he didn't know if he was even able to age at this point.
Water warlock provided him many benefits that allowed him to take risks no one else could, but at the same time it added a whole set of different dangers. Being eaten alive, being trapped or anything that could potentially get him stuck for eternity.
Still, as he fell into the dark pit the only think he could think of was of his friends and how even risking being trapped forever was nothing compared to the thought of them dying.
