Well i got it to work almsot the way i wanted thanks to you boys. Here's what I did:
1-copied the impale ability named it impale A
2-set air time to 1.5, damage to 100, AOE to 150, Range 600, duration 0, hero ability false, targets enemy-ground-organic.
3-copied rifleman unit, named it dummy(lurker), gave it the impale A ability, Has water shadow false, Model file- none, Shadow image (unit)- false, attacks enabled none, movement 0, food 0.
4-trigger
EVENTS-every 4 seconds game time
ACTIONS-Unit Group - Pick every unit in.....and do actions
LOOP ACTIONS-IF\THEN\ELSE
IF-CONDITIONS
(Unit type of(picked unit)) equal to burrowed crypt fiend
THEN-ACTIONS
Unit- create 1 dummy(lurker) for (owner of picked unit) at (position of (picked unit)) facing default building facing degrees.
Unit - add 0.5 second generic expiration timer to (Last created unit)
Unit - Order (last created unit) to Undead crypt lord impale (random unit from (Units within 600 of (position of(last created unit))matching ((((Matching unit) is alive) equal to true) and (((owner of (matching unit)) is an enemy of (owner of(picked unit))) equal to true
ELSE ACTIONS
do nothing
Originally i had a wierd bug where the lurker targets would sometimes be turned invulnerable if the lurkers were to close. I noticed that when the targets are flying in the air, they are turned invulnerable so i figured that it had something to do with the creation of the dummy units and them interfering with the landing of the lurker targets so I seemed to have gotten rid of that problem by extending the air time, increasing the interval between computer checks in the events line of the trigger (from 3.5 seconds to 4) and decreasing the length of the expiration timer for the dummy (lurker) from 2 seconds to .5. I also couldn't get the lurker to actually do damage to buildings. It would attack buildings but wouldn't damage them. I noticed that damage to regular units only occured after they had landed, and that when i set the air time to zero, they wouldn't get damaged at all so I figured that unless the buildings would get throw up in the air, they would never get damaged. I also played around withDrain Pipes third suggestion of using the slow orb ability but I found that his second suggestion (the one used above) seemed to work better. I've now got it to a point where it works pretty good with issues minor enough that it won't prevent me from using it (lurkers attack invisible units even if they're not detected and they still attack buildings but don't damage them). Thanks alot for your help boys and happy new year (my new years hangover is finally gone
).