Activate Phyrexian Altar by sacrificing Riftsweeper, adding one mana of any color.
Riftsweeper dies, triggering Verdant Succession and Myrkul.
Resolve the Myrkul trigger, causing you to exile Riftsweeper and create a token copy of Riftsweeper that is an enchantment and loses all other card types.
The Riftsweeper token enters the battlefield, shuffling the nontoken Riftsweeper into your library from exile.
Resolve the Verdant Succession trigger from step 2, searching your library for Riftsweeper and putting it onto the battlefield.
Riftsweeper enters the battlefield, shuffling any exiled card into its owner's library.