A desync is when a player "desynchronises" with the host - in other words, the game on the player's computer is at a different point to the game on the host's computer. This can cause "game splits" - where there are effectively 2 games running at once - or disconnection.
And how to make a dummy unit cast modified curse on it? Well...
1. Go into the object editor and create a custom ability based off curse. Reduce the mana cost to 0 and change the chance to miss to 100%. Also make it have 3 levels with different durations - 5.00, 7.50 and 10.00.
2. Create your dummy unit - this is basically a unit with a tiny scale size. Add your new curse ability to it, and also add the ability "locust" so it can't be selected.
3. In your trigger, create a dummy unit at the position of the target.
4. Set the level of your curse ability for the dummy equal to the level of the spell for the caster.
5. Order the dummy to Undead Banshee - Curse the target of the spell.
6. Add an expiration timer to the dummy unit with a duration of 0.5 seconds.
EDIT:
Ah, Mephisto, I do believe you have an easier option... ignore the above instructions, just make the base ability for your spell curse with 100% miss chance