| NateMg |
There would typically be a perception check to see if we can spot an ambush preceding combat.
If the dog was aware of Darius while Darius was unaware of the dog (i.e. it is lying in ambush), we could be in a surprise round in which case, Darius would be flatfooted despite rolling a higher initiative.
However, in a surprise round the dog would only get a move action or a standard action. It would not be able to move and bite as your description states.
Absent a surprise round, the combat would kick off with "?" reducing Darius but under no circumstances should he be bitten.
Also note that Reduce Person with a casting time of 1 round cannot be cast in surprise as it takes more than a standard action.