"Conan the Conqueror" by Robert E. Howard

This is another of the Conan editions edited by Sprague LeCamp. In this novelette Conan is around 46 and has been a king of Aquilionia for a while.  He got there by deposing the original kings and their dynasty and in this volume Valerius, one of the heirs of the old dynasty, with the help of a wizard awakes a 3,000 year dead wizard Xulthotun and with his help win over Conan's army and overthrow Conan as the king of Aquilonia.  However Xulthotun secretly saves Conan and brings him to the siege camp to offer him to be the puppet king of Aquilonia instead of Valerius, but Conan refuses and is saved from a death by being eaten by a giant, gray ape by a slave girl.
  Conan then goes through much of the new kingdoms gathering support for himself but also trying to follow the path of the Hearth of Ahriman, a magical jewel that can be used to bring mummies back to life and fight against the magic of Xulthotun.  Conan ends in Stygia, way south, where the cultist of Seth, the snake god, have stolen the jewel to put it in a pyramid-shaped temple of Seth and bring back to life a number of ancient Stygian sorcerers from their mummies.  Conan is kidnapped by a 10,000 year old near-naked female vampire, but he resist her call and eventually kills the cultists and the Kithai followers and gets the jewel.
  On the way back to Aquilonia Conan gathers a huge army and attacks Xulthotun and Valerius near a river which fails to rise as cast by Xulthotun's magic because the Heart of Ahriman was used against him.  Finally Xulthotun is killed by Conan and turns into a shriveled mummy, which is what he was all along.  Conan becomes a king of Aquillonia once again and frees the slave girl that helped him.
  Overall an interesting read, though too much emphasis on battles and less on monsters, which is what usually makes good Conan stories.  Anyway, written by the legendary Robert E. Howard and published last before his suicide at age 30, this is an entertaining and exciting story in the Conan storyline.