Imperious Knights are only found in the Undead Crypt. In NG, there will be two of them at the end of a hallway after the Nameless Usurper invades. In NG+ and beyond, there will be two more in the large room with the three Leydia Witches; these two don't respawn.
These enemies are armored knights that wield a pair of interlocking greatshields, Orma's Greatshield and Reeve's Greatshield, as weapons. There are only two encountered in the Undead Crypt, and another two will be encountered when in NG+.
The Imperious Knights were former warriors who attempted to conquer the Undead Crypt. However, they failed in their attempt and were slaughtered by the Grave Wardens, who used their enslaved souls to guard the crypt from intruders.
They will keep their shields up until attacked a few times. Once provoked, they attempt to bash the player with their greatshields, leaving them open to counter-attacks. Some effective strategies are to attack from behind their shield where they are not protected or to backstep and attack with a ranged weapon.
They drop the following at drop rate of 5%.