As you might notice, the images created in Trance do not deal any damage and are harmless to enemies.
Trance aims to lead your enemies to wrong decisions. What do you think how scared he will be when you use Trance Move to teleport an image right behind him ? he will spam that image with spells, causing cooldown of the casted abilities and give you a chance to take a step!
Spamming images might be a good idea to hide yourself under them, as he won't be able to differ between your images and your actual hero. Just create a few, go off Trance and get him!
Be careful you don't stay too long invisible, as, to make this spell not overpowered with permanent invisibility, the Trance will deactivate itself after 8 seconds of non-casting. I added this to prevent, as I said, the hero being overpowered and to bring some balance to the ability, as the movement speed reduction does, too.