They give bonuses to Dragon Princes and Dragon units meaning that such units should make up the bulk of your elite army. The Knights of Caledor play similarly to other High Elf factions, but with a heavier focus on their most elite units. Imrik has inherited his family's affinity for dragons.
Imrik is descended from two Phoenix Kings (Caledor I and Caledor II) as well as Caledor Dragontamer, companion of Aenarion, who first woke the dragons to fight against Chaos. While he is away, he has entrusted command of Caledor to others. Sufficed to say, Imrik may be under the blazing hot Nehekharan sun, but he’s going to make his opponents experience a living hell.The Knights of Caledor are an expedition led by Prince Imrik, the ruler of Caledor in Ulthuan. By level 14, he’ll get his unique dragon mount Minaithnir which, according to Total War Reddit user UlrikHD, will turn him into the highest DPS legendary lord in the game - having a free Star Dragon that can breathe celestial flames definitely helps. This is bolstered by Blazing Lance (also obtained at level 12) which increases your charge bonus and allows attacks to cause the flammable effect (additional weakness to fire). Likewise, you can get Ashen Fields (increased weapon strength and 20% weakness to fire damage for all units in local enemy armies). It has awesome buffs like Dragonheart (boosted fire resistance for all units, leadership, and melee attack) and others that increase the capabilities of cavalry units like Dragon Princes. His Caledor Incumbent unique yellow line of perks becomes available at level 12.
As for his skills, Dragonhorn increases melee damage and can cause fear, and it has a map-wide effect for all friendly units.