Endarken is a verb that means to make dark or darker. 

You can think of the word endarken as a counterpoint to ‘enlighten.’ Both take words of Old English origin and add the prefix E-N which expresses entry into a specified state or location. Here’s an example of endarken in use:

Every time we start talking about camping our friend Sam seems to endarken the conversation by talking about all the dangerous creatures lurking in the woods. That guy has a way of making almost any conversation needlessly dark.