I think you are looking for action type 50 (SMART_ACTION_SUMMON_GO) on your npc, tied to whatever trigger event you want. Action 50 has a despawn parameter you can use. From there you just need to script the GO.
I’m not familiar with spell id 7777 (if that is the actual spell id and not just a placeholder) but if the spell already has a built effect (like a damage aura) then you could just use action 12 (cast), if the spell activates the aura, or action 75 (add aura) if the spell is already an aura. Again pending on how the spell works you can either use the in-combat (event 0) , out-of-combat (event 1), range (event 9), or Summoned_Unit (event 17) event to trigger. Don’t forget that GO use source_type 1.
I haven’t done a lot of GO scripting so you might have to use a timed action list to get things to work right. Could you use an untargetable npc instead of the GO, smartai has better support for creatures?
Trinity’s Smart AI page is fairly detailed and should help.
