[AreaOfEffect]

Your thugs have nature magic. The spell to cast is Elemental Fortitude combined with items like the frost brand.

The frost demons are called Ao-Oni. They a recruitables for the nation of Yomi, so seeing them is a matter of starting a new game with the nation and pressing 'r'. These units become summons in later ages.

In regards to Yomi being a pushover or not, the truth is that any nation can be a challenge with the right player set behind it. All nations have tools to deal with any other nation, and if they don't, well that is what pretenders are for. A pretender can always lend any nation things they do not naturally have, thus give them new advantages. Sure some nations have natural advantages over several others, and some nations are just easier to conquer with (looking at you skinshifters), but all nations have a chance to 'pwn'.

Edit: Also, I would not rely on the AI to cast spells that you want. I would also not buff a thug and then set them to cast spells at all. You buff a thug so that they can wade through melee. Casting spells yields too much fatigue in the end and will cause your units to die. Fatigue lowers attack and defense skill, it also allows for a greater chance of critical hits, making your units armor nearly worthless. Instead you should have ordered your units to: buff, buff, buff, hold, hold, attack. Holding allows your thugs to recover fatigue from the buffs. Either script your units to cast evocations and the like on the enemy before they reach you, or cast buffs and attack. A mix of the two will generally get you killed.