As I mentioned in the "
A return on Conceal Spell" thread, Conceal is indeed a Stealth ability. What you're trying to do is transform it into another combat ability because you don't want it to be
just a Possession ability, which is not necessarily a bad goal. However, good intentions won't make the game more balanced. Without an effective way to change Conceal into a useful and balanced combat ability, you'll make it worse than before. There's just too many problems with what you're trying to do.
The first major issue is simply a question of balance with the Keeper variation of the spell. Obviously a casting on a Horned Reaper or a different offensive Creature like the Hell Hound would prove itself much more effective than using it on other Creatures. With such Creatures, it becomes rather abusable. Another abusable tactic is simply refreshing the casting on a Creature over and over again. The costs may add up after a while, but with a strong offensive Creature, 15 seconds for 500 Gold is quite a bargain. Additionally, utilizing Gold Costs as a balancing trait will result in similar effects as to the Lightning Spell. Players trying to play "fairly" may likely find such abusive opportunities discouraging as it will be difficult to judge where they're crossing the line between fair use and abuse.
You can't really draw a line between balance and fairness with this sort of thing. You have to take in consideration how the offensive Creatures benifit compared to other Creatures. You can make it very costly to fit it better for the more useful/offensive Creature castings like the ones previously mentioned, but then it becomes more worthless with just about every other Creature. Doing the opposite just leaves an easy opportunity for very abusive tactics. There's not much of a "middle line" because the best you can do is make it so it's noticably overpowered on offensive Creatures and noticably underpowered on others.
The second main problem relates to technical difficulties. The AI simply does not handle it very well. I won't go too into the details here because I know you also know of this issue, having brought it up before. However, I will mention something I noticed that you may not be familiar with, which is that if an invisible Creature attacks someone during battle, said attacked Creature may actually start
fleeing from battle. I've suffered with these issues in my own patch previously with just the Thief having a permanent Invisibility. With a large number of Invisible Creatures, this may become a bit more problematic. Heroes can prove themselves to be especially annoying, as their AI will have them seek out Imps with highest priority.
Lastly, the problem with the AI also contradicts with your goals. You're trying to expand it beyond just Player usage, yet enemy Keeper AI also does not know how to handle the spell properly. This generally defeats the purpose as in the end, it will be essentially a Player only Spell.
Unless you can find a way to effectively solve these issues, I would keep Invisibility as it is.