Documentation at https://melpa.org/#/hydra

This package can be used to tie related commands into a family of short bindings with a common prefix---a Hydra. Once you summon the Hydra (through the prefixed binding), all the heads can be called in succession with only a short extension. Any binding that isn't the Hydra's head vanquishes the Hydra. Note that the final binding, besides vanquishing the Hydra, will still serve its original purpose, calling the command assigned to it. This makes the Hydra very seamless; it's like a minor mode that disables itself automatically.