To start a dialog when player enters a trigger you could use this in a story script:

Code
IF
CharacterEnteredTrigger(_Player,_EventTriggerName)
THEN
SelectAndStartDialog(_Player,_NPCName);

Or
Code
Proc_StartDialog(0,"DialogName",_NPCName,_Player);


To a start combat if a player choses wrong answer you could do (after you set the flag in a dialog node)

Code
IF
DialogEnded("DialogName", _)
AND
DB_IsPlayer(_Player)
AND
ObjectGetFlag(_Player, "FlagName", 1)
THEN
SetFaction(_NPCNAME,"Evil NPC");