Spell: Unit leaves posion trail

So how would i need to make that ? I have few idea, creating dummy with immolation ability, but i dont know how to make detection system that detects if there is any posion dummy in range of all posion dummies, like when posion is spawned, there wont be next one spawned until you leave some range of posion cloud (any).

Did you mean "Poison?" I would simply suggest you creating an invisible locust dummy with immolation every one second of the game, and add an expiration timer to them of around five to ten seconds.

-.-', y but, when unit starts ability, berserk based; then he should drop poison on the ground where he is walking, but it must be disabled for him to drop poison on the position where he dropped it already. Must be a range between them.

What difference does it make? The units expire anyways... You can always add a check for a dummy unit type around the unit before creating it.

When the unit starts the spell, start a timer that goes off every second (or whatever), and then in that timer callback check if there's a poison cloud in X range. If not, make one with an expiration timer. Once the spell ends (best to check by buff in case of early removal), just stop the timer. It's really easy.