Drowning
Belialthorn didn't have time to scream or do anything else as he was dragging into cold water. His survival instincts kicked in and he trashed, his legs kicking out but all that did was make whatever was holding him tighten its hold.
He opened his eyes, maybe he could see what was holding him before his lungs gave out but the lake was now dull green, and particles of brown gloop were floating around, making it difficult to make anything out.
His lungs screamed at him and he felt his consciousness starting to blank off, his mind scrambled to put together a spell but it kept slipping like trying to catch the wind with your hand.
He finally succeeded in casting one spell, but it was a fire one that was doused before it even manifested. The realization that he would die hit him harder than the thought of been pulled under.