The hybrid runtime (HRT) model offers a path towards high performance and efficiency. By integrating the OS kernel, runtime, and application, an HRT allows the runtime developer to leverage the full feature set of the hardware and specialize OS services to the runtime’s needs. However, conforming to the HRT model currently requires a port of the runtime to the kernel level, for example to the Nautilus kernel framework, and this requires knowledge of kernel internals. In response, we developed Multiverse, a system that bridges the gap between a built-from-scratch HRT and a legacy runtime system. Multiverse allows unmodified applications and runtimes to be brought into the HRT model without any porting effort whatsoever by splitting the execution of the application between the domains of a legacy OS and an HRT environment. We describe the design and implementation of Multiverse and illustrate its capabilities using the massive, widely-used Racket runtime system.