Answer:
For in that sleep of death what dreams may come.
Step-by-step explanation:
A main clause is an independent clause that has a subject and verb and has meaning of its own. For example, I kicked the ball or I will run later.
Likewise, the main clause in the following sentence is "For in that sleep of death what dreams may come." The 'sleep of death' is the subject while 'may come' is the verb phrase.
The meaning of this sentence is that there are endless possibilities of what may come after one decides to leave everything behind the past. It is assuring a new journey with different opportunities.