The bottom of sceptre is one of 4 parts of a skull sceptre. It can be obtained from Flesh Crawlers in the second level of the Stronghold of Security. The other parts of the sceptre must be obtained from a minotaur, an ankou, and a catablepon. As with the other parts of the sceptre, this item cannot be traded. The bottom of sceptre can be combined with the top of sceptre to create a runed sceptre. After combining a message in chatbox will state: "The two halves of the Sceptre fit perfectly. The Sceptre appears to be designed to have something on top."
For an exhaustive list of all known sources for this item, see here