ScriptEffectStart
Usage
begin ScriptEffectStart
Special block type used only by Magic Effect scripts. This block will run on the first iteration of a scripted spell effect.
Note: Any Script Effect spell will run on the target Actor when they are hit with it. This means that the default reference for the script is the Actor that has been hit by it, not the player (though the actor that has been hit, could be the player). Also it means that if this block runs, then the spell has just hit the Actor and so a ScriptEffectStart can almost be considered an OnHitWith <Spellname> block.
The fact that Script Effect Spells run on whoever has been hit by the spell, basically gives the programmer the ability to apply a script on any Actor in the game for a period of time, and can be used for getting Actor reference and many other effects.