"Master, I strongly urge you to not do this!"
For the first time in his life, Talos yelled at Arnold and he wasn't the only one on to do so either. All of Arnold's Elite summons were trying to make him change his mind about entering within the world tree's range. Even Tiamut was amongst the ones trying to persuade Arnold to walk away.
However, her worries were mostly because of Scylla and less about Arnold's safety. She knew the Dragon referred to Arnold as her parent and if anything happened to him... she would get to know instantly and all hell would break loose over the planet.
Then there was Cetus. No one truly knew what she would do when her new master dies. As usual, Cetus would change her owner and adapt to their wishes accordingly. Thus if the World tree actually managed to kill Arnold... Cetus would then become used to the tree's consciousness and then might even do something she had never done before... to harm the humans.