8a: The river can be sourced from the Russian mountains. 4 letters. ?r??
2d: O, so this is an in- group? 6 letters. ???q?e. Is it unique??
25a/3d: or setting on an unearthly dimension perhaps? 5/6 letters. ?????t and ?e?e?
19d: Looks like a positive sign, if it deflects Dracula. 3/5 letters. T?? and ???s?
20a: What a holy show, by Jeremy. 4 letters. ?y??