The Pleiades in Overlord, otherwise known as the Combat Maid Squad of Nazarick, are secondary characters compared to Ainz and the Floor Guardians. However, they're still quite powerful. Especially so when a reader/viewer compares them to anyone else in this Yggdrasil-like world, as they all tend to average between levels 10-30.
The Pleiades aren't quite as much of a jump in power or level as the other Nazarick members, but these battle maids do have levels that range from the late 40s to even the max of 100, and each has a different specialization. Additionally, a lot of the Jobs, Races, and game-like mechanics of Overlord's setting haven't been revealed yet in the source material, so this ranking of strength is based on information provided in the story thus far.
Slight spoilers for the Pleiades members in Overlord below. Thanks to SkytheWood for the Character Sheet translations.
8 Entoma Vasilissa Zeta: The Entomancer
Sadly, taking up the rear of the group is likely the most visually distinct maid, Entoma Vasilissa Zeta. While Entoma may seem absurdly strong, especially when viewers see how she fights, that's only in comparison to Blue Rose members. Even Evil Eye, the strongest member, was able to take her down thanks to having an absurdly lucky Spell called Vermin Bane, which works incredibly well against insects. The situation may have been against her, but Entoma still genuinely lost against a non-Nazarick native. No other Battle Maid in the anime has suffered a defeat like this before.
Entoma is primarily part of the Arachnoid Race, among others, and has surprisingly specialized in her Job Levels more than her Racial ones. She's mostly specced into being a Talismancer and Entomancer, which are actually more suited for Support than straight Offense. As a final nail in the coffin that puts Entoma at the bottom of the rankings, she's only level 51, which is near the lower end of Pleiades levels.
7 Solution Episilon: The Slime-Shoggoth
Solution is the tattle-tale torture-loving Shoggoth of the Pleiades. Solution, in contrast to Entoma, is much more specialized in her Racial Levels than her Job Levels. She's a 10th-level Shoggoth, Ubbo-Sathla, and a few other undisclosed Races. Basically, she's a truly evil Slime monster. However, her highest level Job is Poison Maker at only 4th level followed by Assassin at 2nd level. Solution specializes in capturing enemies by absorbing them into her empty, yet seemingly infinite, body, where she can choose to either digest them or keep them safe. She's essentially a Slime that has all the stereotypical benefits of this monster Race — as well as their weaknesses, like her weakness to Blunt-type damage.
This glaring weakness to a common damage type, low levels in her Jobs, and specialization towards capture and espionage are the main reasons why Solution is ranked near the bottom. However, she's another character that Overlord fans haven't seen fighting much either.
6 Yuri Alpha: The Striker
Speaking of Blunt-type damage, Yuri Alpha is the Dullahan Race and Striker Job-user of the group who specializes in close-combat direct damage using Fa Jin-type skills. She's much more of a direct damage dealer than Entoma is. That said, even though she's vice-captain of the squad, Yuri's level is on the lower end at 51.
Still, most of the other maids treat Yuri like a "big sister" of sorts, regardless of her rank or tier. It's obvious that she's well-respected for her professionalism and intelligence. As one of Yamaiko's creations, Yuri uses Gauntlets as her creator did. She also seems to have a similar sort of personality, which is likely why she's also one of the only characters that doesn't innately "hate" humans, along with Nigredo and Pestonya.
5 CZ2128 "Shizu" Delta: The Gunner
It's a bit surprising to see CZ ranked this high, considering she's the lowest level of the Pleiades at 46, but there's a reason for this. For one, Delta is an Automaton, meaning she's innately resistant to a lot of damage types that the other maids would struggle with. Secondly, she's almost entirely specialized in her Jobs of Gunner and Sniper, which makes her unstoppable if she fights at a distance.
And, considering she also has a good amount of levels in the Assassin and Stalker Jobs, CZ is great at avoiding her enemies until she's in a perfect position to blast them away from afar.
4 Lupusregina Beta: The Battle-Cleric
Though fans haven't seen it in the anime yet, Lupusregina Beta is a Werewolf and also fills the role of "Battle Cleric" in the Pleiades. Like CZ, she only has five levels in her Racial Level, and the rest are put into her Jobs. She has 10 levels in Cleric, 5 in Battle Cleric, 5 in Hierophant, and 4 in Warlord, as well as 30 other levels in various unnamed Jobs.
Lupusregina is the only maid with a way to heal herself a great deal in combat, allowing her to outlast most of her opponents in a fight. She's a bit more "mundane" compared to a Dullahan, an Automaton, or other monster-type characters in similar anime. However, she's absolutely one of the stronger Maids of the Pleiades.
3 Naberal Gamma: The Mage
Nabarel Gamma is the battle maid that fans of the Overlord anime have likely seen the most, and she's also the least "emotional." But, as the highest-leveled member of the Pleiades at 63, Naberal isn't really afraid of much. Nabarel is another maid who has heavily invested in her Jobs, with only a single level in the Doppelganger Race and the other 62 levels being split among War Wizard (10), Air Elementalist (10), Armored Mage (10), Fighter (1), as well as a ton of other unnamed Jobs.
She's the main "caster" of the squad, using a ton of different Elemental magics to deal massive single-target or AoE damage. And, considering she's the one that Ainz brought with him to infiltrate E-Rantel, Naberal is strong enough (and scary enough) for even the great ruler Ainz Ooal Gown to acknowledge her.
2 Aurole Omega: The Commander
Aurole Omega is a member of the Pleiades that almost no one knows about. That's because she typically stays in Nazarick as an Area Guardian of the Cherry Blossom Sanctuary. From here, Omega manages all the teleportation gates in Nazarick. She also protects the Staff of Ainz Ooal Gown whenever Momonga isn't using it.
As a fighter, however, she's quite unique in that she's one of the only "humans" who are a part of Nazarick. Sadly, tfans haven't seen much of Aurole Omega's power even in the original Light Novel, so it's hard to say confidently how strong she is. But, as a level 100 Area Guardian with a variety of levels in Commander-type Jobs, it's safe to say that she's likely very strong.
1 Sebas Tian: The Ki Master Leader
Technically, while Sebas isn't a "Battle Maid" (obviously), he is still the leader of them. And, even though he's also not technically a Floor Guardian like Demiurge or Aura, his power is more closely comparable. Similar to someone like Yuri Alpha, Sebas is an unarmed specialist who uses punches and kicks to blow away his enemies.
His Job and Race levels are at a 3 to 1 ratio between them. He has 75 out of his 100 levels put into his Jobs. 15 are in the Ki Master Job, 10 in Martial King, 10 in Monk, 5 in Striker, and 5 in Out Ki Master. As the leader of the group, a level 100 NPC, and an offense-type fighter, Sebas is by far the strongest of the Pleiades who viewers have likely seen fighting the most out of the group.