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All Tier 5 units have their uses...
Let's start with Nightmares: When clearing the map, I create several stacks of Nightmares and use them to chase down neutral creatures. If Fear works, that's great, but even without it, they have excellent speed, which is perfect for blocking ranged units. Against factions with strong ranged units, my choice is Nightmares because they can quickly destroy the enemy's defensive fortifications and expose their ranged units. However, against factions that don't rely heavily on ranged units, Hellhounds with their Withering Aura are more effective. By the way, it's best to use Nightmares and Hellhounds to open portals – their abilities will definitely pay off. One drawback is their Aura of Terror, as they are already quite large, and it extends to three squares around them. I haven't noticed any other drawbacks. In my opinion, it's the best Tier 5 unit.
Archliches and Elder Liches: As I said, they work best in combination (one shoots, the other revives). On the other hand, against factions that will protect their ranged units, you should only buy Archliches so that they can eliminate both the ranged units and the units protecting them, especially effective against Succubi and Imps (if you shoot at a unit adjacent to them, they will take damage but won't retaliate), and they can also cast a decent Plague. However, there is a significant drawback: they occupy 4 squares, which is very detrimental for a ranged unit. (Centaurs maneuver, and Titans don't have a penalty in melee combat), so it's difficult to protect them, and if the Elder Liches die, you're in trouble.
Various Hydras: I completely agree with Mdanse – they are very durable, attack with six, generate Hellhounds (I've saved myself many times with this), and what about their speed? Do they even need speed? They are a defensive unit – Ents are slower, and no one complains about them. When everyone rushes towards the Black Knight, that's when they shine, and they can also launch a surprise attack. Everyone probably remembers what Letos used to do with Hydras on dueling servers. Also, I've noticed that when playing against the AI, its ranged units really like to shoot at Hydras, while valuable units like Fury live a little longer.
Inquisitors and Adepts: Honestly, it's embarrassing – Rampart Inquisitors already shoot... I understand everything, but this is a complete failure. Wouldn't it be better to transfer them to Paladins, leaving a couple to cast Haste on Archers, Stone Skin on Paladins, or Adepts to cast Power Attack on Paladins and remove enchantments? In my opinion, it's the worst Tier 5 unit.
Unicorns: Simply adorable – they stand behind at first to provide resistance to ranged attacks and protection from four-square units, and then they move forward and act as a workhorse. However, you need Soldier's Luck – otherwise, the Blind effect will start to occur every other time. In my opinion, it's the second-best Tier 5 unit. The only problem is that building them can be difficult – you have to choose between them and Dragons.
Djinn: The best ranged unit suppressor – it flies far, hits decently, and only has a few hit points. And by the way, why does it seem to be better at using Dark Magic than Light Magic? It's strange, but it often casts Plague and Weakness on enemies, while I usually cast Stone Skin on my own units. However, I don't like the Djinn's Luck ability at all – it's not very convincing. If it guaranteed to increase or decrease by 2, that would be great, but as it is, it's better to just attack with them.
I can't say anything about the Barbarians – I don't even know who their Tier 5 unit is. If it's the one that attacks twice, then they are decent fighters. If it's the Shamans, then they are not very good.
And I can't talk about Priests – I have a biased opinion about them
