Please help with a spell

Please help with a spell

Please I need help with a spell. This is the description:

Shoots a extremly frozen arrow to the sky (350 casting range) at the point. The arrow falls when appears an enemy hero, chasing him/her for a time, dealing damage when snag the hero. Every 0.5 seconds after the arrow chase the hero, it increases its damage until time overs or hero takes the damage of arrow. Arrows are invulnerable and has 325/350/375 movespeed, can chase invisible and invulnerable units. Max of X arrows.

Note: arrows can't be near, at least a 450 range for each. spell disabled when max numbers of arrows are summoned.
-Chasing arrow lose the hero and dies when the hero is in +900 range.

Level 1 - 100 damage, +25 damage every 0.5 seconds. Max of 2 arrows. 30sec duration of arrows.
Level 2 - 200 damage, +50 damage every 0.5 seconds. Max of 3 arrows. 45sec duration of arrows.
Level 3 - 300 damage, +75 damage every 0.5 seconds. Max of 4 arrows. 60sec duration of arrows

Explanation: I like to make an invisible summons that is triggered when a unit enters 200 range. An arrow will be created and starts to chase the unit, every 0.5 seconds the damage is increased. Please help

u must use the carrian beetle spell so it has a max number of summons. just create the arrow dummy (not a dummy that shoots and arrow, but a real arrow unit) and when u create it use 'unit - hide unit' when a hero comes near unhide it and let it follow the hero. also activate this trigger

events: every 0.5 sec of the game
conditions: --------
actions: set 'an integer variable) = (airthemathic) XXX (base arrow damage)+ XXX (the damage it gets every 0.5 sec

when the hero gets hit by the arrow, turn the trigger off