Order of actions in Ascension
The Storm of Souls rulebook says:


  When an Event card flips into the Center Row, immediately move it to the Event Zone on the board, and then replace it in the Center Row.
  [...]
  If a new Event flips, the old Event immediately goes to the Void and the new Event becomes the current event.


The keyword is immediately. Therefore, if Souls Unbound enters the Center Row right after you kill Mephit (or any banishing Monster), you can only banish Heroes and Constructs from the Center Row.

The iOS version of the game follows the same logic.