is this possible?

is it possible to make a trigger so an event will occur when a monster is killed?

Yes, your event should be, a unit comparison. And it should say when (select unit) dies
then have your action be whatever event you want to occur when that unit dies

ok i think i have it now, what im trying to do is have a large snow storm when the dragon in the middle dies.

Ok well then what you need to do is somewhere off the map, Like on the dark sides where you can still create stuff, make a region and have it snowing in it. Make it as big as you want the region aroudn the dragon to be. Then when the dragon dies, use the previously mention trigger, but for action have it be
Move Region to center of triggering unit

is it possible to add more values to an event? because all i have so far, is
unit: red dragon
action: dies
or do i need to add a new event and link them somehow?
this is my first try at modding anything btw

the red dragon part is fine as long as it is the only red dragon on the map. If there are others, then everytime you kill a red dragon, the snow will appear. If you have more than one red dragon, you need to select that specific red dragon as your unti. So instead of picking the red dragon from the unit list, choose the middle option in the box, click choose unit, or select unit. It will then bring you back to your map where all you have to do is click on the red dragon you want the snow to be around when it dies. Its kinda hard to explain, I just hope you only have one red dragon. Maybe someone else can explain it better? But what you are trying to do is very doable.

guys, just amke a trigger

event- unit dies
conditions- whatever
action- create weather effect at location

there:D

Ok so that is the way to do this in simplified form, if you didnt notice that the person asking question is new at this and needs more in depth than just a 3 line explanation. But thanks...