Craft 3 RC Upgrade killed the WYSIWYG Editor
With RC1 the rich text editor was moved to a plugin. You can find the Redactor editor (the previous richtext field) in the plugin store along with a new CKEditor editor as well.

You can read about this in the docs, too: https://docs.craftcms.com/v3/changes-in-craft-3.html#rich-text-fields