The simplest way is an I_C_T in Nimbul's dialogue, it sets a variable you can later track:
I_C_T NimbulDialogue 999 PrefixVariableNimbulTrigger
== MyNPC IF ~InParty("MyNPC") InMyArea("MyNPC") !StateCheck("MyNPC",CD_STATE_NOTVALID)~ THEN ~Myahaha, now I can trigger dialogue via PrefixVariableNimbulTrigger, because it'll be automatically set to 1 after this interjection!~
It has the obvious drawback of only triggering if MyNPC is in party at the moment - but then again, it may be just what you need. I hesitate to offer other means(adding a death variable, altering Nimbul's script if he doesn't have a unique one, or adding ALTER_TRANS, because it may kill BG1 NPC and other interjections, just like with Mulahey).
If Nimbul DOES have a unique script, like MyNimbulScript, it's even easier: either find a unique variable in his script and use it, or add this block to his script:
And use MyPrefixVar later.