To create an effect with an offset in relation to the ground before 1.30 patch, see http://www.hiveworkshop.com/forums/1561722-post10.html
In case of 1.30 patch or higher, use BlzSetSpecialEffectZ
native.
To create an effect only visible to one player see https://www.hiveworkshop.com/threads/gs.300430/#post-3209073
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Creates the special effect in point with coordinates (x;y) using the model file with a path modelName. The altitude (Z) of the newly spawned effect is at the ground level, be it terrain, some pathable destructable or on top of water. In other words, the effect's Z coordinate does not have to be 0.