Now that every other source of invisible units are unavailable in the very early part of the game in the next version, it's time to consider invisible Air Elementals again.
There are three parts to consider.
1. Where do you obtain them. As Rampaging Monsters, they can appear as early as turn 40-50 unless the difficulty is very low. In lairs themselves, they are available from the beginning. Converting them to Undead can be done fairly easily using the Syphon Life spell, or if their resistance is lowered, Life Drain as well. Ghouls can also do so, albeit probably need many to be able to deal enough damage, or need focus magic. Zombies are extremely unlikely to be able to convert one. Nonetheless, both the availability of the monster and the spells to raise them exists in the early game.
2. How bad is it. Unlike invisbile heroes, an Air Elemental is super fragile, can't shoot and generally takes damage in melee at least enough of it to die in a few battles. If the other player has any way to detect the Air Elemental, they'll be able to use Fire Bolt, Star Fires, AEther Sparks, Psionic Blast maybe Life Drain to damage it in the early game. Nature is out of luck here as it has no invisibility detection and the undead are immune to the cold spells. So Nature wizards pretty much lose to as many as one Air Elemental - you can use combat spells to kill everything, through multiple battles if needed - while other realms can deal with it. However, lairs or nodes can't deal with it, so having as few as one invisible unit opens up the possibility to clear out otherwise very difficult lairs such as storm giants, efreets, water elementals, stone giants, or pretty much anything ranged. It also helps a little against larger groups of melee monsters where losing a spearmen on turn 2-3 of combat is guaranteed otherwise. Sure saving the lives of spearmen is no big deal but producing them and transporting them to the lair can take a while and the elemental can do it instantly. Furthermore, the elemenal moves 5 on the overland map, so it can reach and clear out lairs extremely quickly.
Overall I think this is a big enough risk to game balance, although not critical (4 out of 5 realms are barely affected, but neutrals affected extremely heavily.)
3. Possible solutions. Adding Death Immunity is likely a bad idea. Both because it's bad for consistency (other elementals don't have it), but even more so because then Death would have no way to deal with them. Sure, they'll have plenty of ways to reveal the elemental, but neither Life Drain, Syphon Life, nor Reaper Slash could damage it, Gate of Hades is too slow to kill one, it's a combat summon so any additional turns allows the enemy to summon another. Wave of Despair could still kill it easily, but it's a super expensive spell and the Sorcery player will be ahead in casting skill so that's not very good either. I'm worried the immunity would make Death even worse against Sorcery and it's already not very good against it at rare or later.
Disabling the raising of the creature in the early game by adding Supernatural has no other downsides (guaranteed damage will not trigger on such a weak unit), but it would look weird and out of place.
Disabling the raising of combat creatures across the board, like the original game, also works but might be excessive - it makes undead Boars, Fire Elementals and Earth Elementals also impossible. They aren't all that great units but at least Earth Elementals can be useful.
Disabling the raising of the Air Elemental only, without any further explanations is also possible. Not a fan of the idea but it does make sense : Air doesn't have a "dead body" left behind to raise so it's at least somewhat intuitive. Except, by that logic, Fire makes no sense to raise either. The final possibility is disabling the raising of the 3 elementals only.
There are three parts to consider.
1. Where do you obtain them. As Rampaging Monsters, they can appear as early as turn 40-50 unless the difficulty is very low. In lairs themselves, they are available from the beginning. Converting them to Undead can be done fairly easily using the Syphon Life spell, or if their resistance is lowered, Life Drain as well. Ghouls can also do so, albeit probably need many to be able to deal enough damage, or need focus magic. Zombies are extremely unlikely to be able to convert one. Nonetheless, both the availability of the monster and the spells to raise them exists in the early game.
2. How bad is it. Unlike invisbile heroes, an Air Elemental is super fragile, can't shoot and generally takes damage in melee at least enough of it to die in a few battles. If the other player has any way to detect the Air Elemental, they'll be able to use Fire Bolt, Star Fires, AEther Sparks, Psionic Blast maybe Life Drain to damage it in the early game. Nature is out of luck here as it has no invisibility detection and the undead are immune to the cold spells. So Nature wizards pretty much lose to as many as one Air Elemental - you can use combat spells to kill everything, through multiple battles if needed - while other realms can deal with it. However, lairs or nodes can't deal with it, so having as few as one invisible unit opens up the possibility to clear out otherwise very difficult lairs such as storm giants, efreets, water elementals, stone giants, or pretty much anything ranged. It also helps a little against larger groups of melee monsters where losing a spearmen on turn 2-3 of combat is guaranteed otherwise. Sure saving the lives of spearmen is no big deal but producing them and transporting them to the lair can take a while and the elemental can do it instantly. Furthermore, the elemenal moves 5 on the overland map, so it can reach and clear out lairs extremely quickly.
Overall I think this is a big enough risk to game balance, although not critical (4 out of 5 realms are barely affected, but neutrals affected extremely heavily.)
3. Possible solutions. Adding Death Immunity is likely a bad idea. Both because it's bad for consistency (other elementals don't have it), but even more so because then Death would have no way to deal with them. Sure, they'll have plenty of ways to reveal the elemental, but neither Life Drain, Syphon Life, nor Reaper Slash could damage it, Gate of Hades is too slow to kill one, it's a combat summon so any additional turns allows the enemy to summon another. Wave of Despair could still kill it easily, but it's a super expensive spell and the Sorcery player will be ahead in casting skill so that's not very good either. I'm worried the immunity would make Death even worse against Sorcery and it's already not very good against it at rare or later.
Disabling the raising of the creature in the early game by adding Supernatural has no other downsides (guaranteed damage will not trigger on such a weak unit), but it would look weird and out of place.
Disabling the raising of combat creatures across the board, like the original game, also works but might be excessive - it makes undead Boars, Fire Elementals and Earth Elementals also impossible. They aren't all that great units but at least Earth Elementals can be useful.
Disabling the raising of the Air Elemental only, without any further explanations is also possible. Not a fan of the idea but it does make sense : Air doesn't have a "dead body" left behind to raise so it's at least somewhat intuitive. Except, by that logic, Fire makes no sense to raise either. The final possibility is disabling the raising of the 3 elementals only.
