SQL Server Case Statement when IS NULL

CASE WHEN B.[STAT] IS NULL THEN (C.[EVENT DATE]+10)   -- Type DATETIME
     ELSE '-'                                         -- Type VARCHAR
     END AS [DATE]

You need to select one type or the other for the field, the field type can’t vary by row.
The simplest is to remove the ELSE '-' and let it implicitly get the value NULL instead for the second case.