This shouldn't be too difficult (I hope). I'll try to walk you through it.
First create Validators of type Enumerate Area, and set the Target Location + to Caster Unit and set the Search filter to whichever units you want the Lurker to burrow for, and set the Validator Compare to Equal to. Next set its Search Count to 0 and Search Area field to Less than Or Equal to whichever range you want the Lurker to burrow when detecting a unit that passes the Search Filter. This will give us a Validator that will come back true if a unit that a lurker can or can't attack is not in range or not present. Next set your Lurker's burrow ability Stats - Flags field to have Auto-cast enabled, and set the Autocast range to whatever Range we used in the Validator Search Area, the Auto Cast Minimum to 1 and the Auto - Cast Filter to the Filters used in the Enumerate Area Validator. This will make the ability Auto-cast whenever a unit that passes through the filter is in range of the Lurker. For the Unburrow ability, set the Autocast flag on, and this time add the Enumerate Area Validator to its Ability - Autocast Validator. This will make its Unburrow able to activate when no units that pass through the Validator's filter are present. I think that covers most if not all of what you would have to do.
