How do I migrate from Rhythmyx to Drupal?
Rhythmyx is proprietary software. You're likely to find very little, if at all, people in the Drupal community that have experience migrating from it. 

The first thing to know is what technology stack it's built on so you can evaluate tools to pull out content. The vendor's website is scarce on tech details. From what I could find, the language is perhaps Java. It's also supposed to have a static content export &amp; pluggable API so that should be able to assist in pulling out data.

On the Drupal side, there's Feeds &amp;  Migrate. Feeds is best for mostly static content migration where there isn't fresh content coming into the site on day-to-day basis (and thus an ongoing need to sync both the existing site and new site while development/testing is happening). Migration is a best when you have many different custom content types, taxonomy, users, and other custom data that needs to synced regularly while development is occurring. 

Whatever means you find to pull out data from Rhythmyx will affect what import tools you use. If you have static structured data (e.g. CSV, XML, RSS Feed) Feeds is the better choice here. If you have an API that PHP can interface with, Migrate will be more useful.