Completely disappeared without a trace. And deserters, who should have been brought back from previous installments of Heroes.

What are you talking about? How did ice creatures suddenly emerge from lava? They should be stone creatures, like cooled lava.

No, they look the same, but they are a completely different breed (they are not related to fire or lava creatures, they just look similar).
For example, they lived deep in the mountains in the north, almost at the peaks, but then the dwarves came (the new ones, who the fire creatures didn't like - on the snow and with fire!!!) and tamed them.
Actually, I initially thought that the alternate upgrade was the opposite of the unit, so that's why it seemed that way.