Lol this is really weird but funny. Basically I asked myself what would happen if I attached a unit to another and then attach a unit to the unit I attached and so on. The answer is that with some units you get a gigantic unit tower.
However it only works with some units - I think only the units that can carry relics. If you try it with other units they still stack but all at the same height.
Basically you run a search trigger that searches for the unit. Also use a counter or something to set a height limit or the tower will stack up forever and age of mythology will FREEZE. In the effects run the attach trigger attaching the same unit to itself.
This is very advanced but if you somehow understand my search triggers this should be worth trying out!
(Yeebaagooon is gonna love this one!)