The Lords of Outer Night are retired gods who once had whole civilizations worshiping them and now only have a handful venerating them. The power of the blood has spread through many offspring.
They were taller than most mortals being at least seven feet in height. They were each dressed in a different version of traditional costume and had each their own signature mask.
They occupy the second to the top level of the Temple of Kukulkan. White Council intelligence reports that the Lords had once posed as gods to the Ancient Mayans, each with their own identity. In the report, however, is not included that this made them more than merely ancient vampires. Esteban Batiste mentions the "Circle" stating: Never will He name us to the Circle, no matter how many prizes we bring into the Court. It's likely that he was referring to the Lords of Outer Night though there is a slim and unlikely chance the Eebs meant The Circle.
Their power is on par with Donar Vadderung's, and, even if waned, it is more than a mortal can face in a challenge; they can incapacitate a man or being, even crush them with the sheer power of their will alone.
In the series
In Changes, Donar Vadderung tells Harry Dresden that the Lords of Outer Night and the Red King are planning a ritual sacrifice for a Bloodline curse. And, they have chosen Chichén Itzá because of it enormous power source. Vadderung demonstrated their disabling power on Dresden—a "raw brute application of the will". There will be a dozen Lords there at the center of their power. The Lords and the Red King will crush Dresden.
The first time Dresden saw the Lords of Outer Night was at the Temple of Kukulkan when Dresden was being escorted to the Red King by Alamaya—"thirteen lone figures" stood on the level just before the top level.
Dresden and his team encounter them at Chichén Itzá after the duel with Arianna Ortega when the Red King reneged on his promise to let Dresden have his daughter. They and the Red King debilitated Dresden and his team. Karrin Murphy broke free of the spell with the help of Dresden and Bob. Karrin sliced the air with Fidelacchius literally cutting the Red King's spell then Bob, per Dresden's orders, followed Murphy to keep her free of the Lords' spells. Their confrontation led to the battle at the Ball Court.
They are confronted again in the Temple of Kukulkan. Murphy slew one of the Lords on the steps up the Temple. Dresden arrived at the top, and after slaying the Jag Warrior sent to kill Maggie, The King and the Lords press their will down on Dresden, disabling him. He could only watch as events unfolded. the Leanansidhe impersonated the one that Murphy had killed, wearing his unique mask and cloak, to enter the Temple incognito and gave aid to Dresden when the time came. The Lords were ended when the bloodline curse was reversed onto them and the entire Red Court was destroyed.
And so, we stand at the dawn of a new age for our kind, the first acknowledged Court this far into the United States. No longer need we fear the wrath of our enemies. No longer shall we meekly bow our heads and offer our throats to those who claim power over us. Finally, with the strength of the entire Court behind us, with the Lords of the Outer Night to empower us, we will face our enemies. And bring them to their knees.— The end of Bianca St. Claire’s speech, while she looks directly at Harry Dresden.
False gods! Pretenders! Usurpers of truth! Destroyers of faith, of families, of lives, of children! For your crimes against the Mayans, against the peoples of the world, now will you answer! Your time has come! Face judgment Almighty!— Murphy, an angel speaking through her to the Red King and the Lords of Outer Night as she held up Fidelacchius.