Complex sentence: When time comes for SEES to make their decision, the Protagonist refuses to kill Ryoji, despite Ryoji's transformation into Thanatos, and the party decides to find a way to fight Nyx.
Split into simpler sentences: When time comes for SEES to make their decision, the player can decide whether to kill Ryoji or not. In the canonical ending, the Protagonist refuses to kill Ryoji, despite Ryoji's transformation into Thanatos, and the party decides to find a way to fight Nyx.