RenderMan for Houdini (RfH) is an integrated interface to RenderMan inside Houdini. It is automatically included in all Houdini products except Houdini Apprentice. Yes, it is available with Houdini Indie as well! RfH is not a plugin so you don't need to purchase or install it separately.
RenderMan 21.0 features are now available in Houdini 15.5 and Houdini 16.0.
To render with RenderMan inside Houdini, we do require the installation of RenderMan Pro Server, the renderer itself. See Installation and Licensing for more information.
More information can be found in the Houdini Documentation regarding RenderMan, the descriptions for Reyes and RSL are no longer valid.
To complete the setup you should follow the directions for Environment Variables.
RenderMan 21 operates purely on the modern RIS ray tracing system. The Reyes rendering techniques and the RenderMan Shading Language (RSL) are deprecated and unavailable in modern RenderMan.