/u/kieslowskifan has a fantastic post on the creation of the Rasputin mythology out of contemporary Russian court gossip and post-Revolution anti-Romanov polemic. It addresses the "power" aspects of the legend
/u/carlton_the_doorman addresses the rumors specifically connected to Rasputin's assassination using Edvard Razinsky's The Rasputin File, a fascinating book based on the massive reports from early Soviet/Revolution interrogations of czarist political figures. One thing to note here is that he acquired some of his sources privately through an auction house, leaving a fragment of a question about reliability--here's a review from a Russian history professor addressing that issue which is nevertheless overall positive.