Well, I have made one button manually (old-scool pixelbypixel pic) and one "steal it" style button:
I would it:
- You can call from back-up slot only limited ammount of units (as you can have there 2000 dragons... but if your leadership can handle only 4, you will get in battle only 4)
- You will pay for it (like other games have - building your own unit costs 100 money, but recruiting it by instant costs 10x more - as time penalty - no waiting, no training, you get "mercenery" - So you have to buy the units first and place it to the back up slots, than if you want them in battle - you have to pay again and even more - first than they will enter combat
- You should call them anytime you want - you are paying - you are the master
I agree to disable it when fighting bosses
I could imagine it as Iron Fist (as Lestas suggested) or even as Necromancy... it could even work differently - If using Necro Call - your reserve units would be considered as "dead" and formula for Raise Dead would apply (so if you have shamans in our backup slot- you will get vampires or necromances... ) again - your limit will be your own leadership.