Question

All human things are subject to decay. And, when fate summons, monarchs must obey​

Answer 1

Your question doesn’t make any sense without its completion so I have added the completion and explanation for it.

The excerpt in your question is from Mac Flecknoe, a verse mock-heroic satire that was written by Jhon Dryden back in 1682.  

Here is the complete version of that excerpt.

“All human things are subject to decay,

And, when Fate summons, monarchs must obey:

This Flecknoe found, who, like Augustus, young

Was call'd to Empire, and had govern'd long:

In Prose and Verse, was own'd, without dispute

Through all the Realms of Non-sense, absolute.”