Hey there,
Your question states: Which aspect of Charybdis frightens the men more? How does the poet’s language convey this fearfulness? Well, it really all depends on what "Charybdis" is saying to the men. This would be a reason why the men would be frighten. So maybe he were to say things like there death could come to life, or things in that sort. And based on this poet's language, this would convey that anything could be fearless to anyone, it just really depends on what you say and on how you say it.
Hope this helps.