Megicula is one of those antagonists in Black Clover who wasn't introduced until much later in the series. However, that didn't stop her from being an absolute menace, or downright terrifying sometimes.
For those unaware, Megicula is the highest-ranking Devil from the Underworld who is associated with Curse-Warding Magic. For the longest time, she had contracted with and resided in Vanica Zogratis, a member of the Space Kingdom's Dark Triad.
In a series like Black Clover, which is packed with a colorful cast of sinister Devils, Megicula stands out not just because of her power, but also for her highly inquisitive and whimsical nature. This peculiar personality trait allowed her to form a disturbingly great bond with her host. It also drove her to conduct countless cruel experiments on sentient beings, whom she promptly killed off after losing interest in them.
So, to answer the titular question, how powerful is Megicula? Frankly, she's definitely one of the most dangerous Devils in Black Clover, probably even stronger than other immensely powerful Devils like Zagred.
While she may not be on the level of the three supreme rulers of the Underworld, she is still an extremely formidable opponent in battle who has the ability to create and inflict curses on anything and anyone.
Moreover, she's tied to two of the three rulers, sharing space with Lucifero and Beelzebub. This should definitely speak volumes about the amount of power she commands, despite not being at the top of the food chain.
Megicula's powers and abilities in Black Clover
As a high-ranking Devil of the Underworld, Megicula possesses an immense amount of magic power. She's definitely among the strongest Devils in Black Clover, seeing how not even a Star Zero-ranked mage of the Heart Kingdom could kill her.
Megicula's magic attributes and battle prowess are equally terrifying. Using her Curse-Warding Magic, she can create and inflict powerful curses with long-term consequences, just like how she cursed Acier Silva and Lolopechka.
Her malevolent curses allow her to cause physical damage to her opponents and weaken enemy spells. Megicula can also use her magic attribute to create beneficial curses, which can be used for healing and providing other types of support in active combat. It's also worth mentioning that her magic power can overwhelm even arcane magic.
Through her connection and control over Vanica's soul, she also has access to Blood Magic - a magic attribute which allows her to generate and manipulate blood.
Additionally, due to her connection to Acier Silva, which gave her control over the latter's soul, Megicula also gained access to Steel Magic. It allowed her to generate and manipulate steel, which she can effectively use to block attacks in battle.

Thus, by having control over Vanica and Acier's souls, Megicula added two different types of magic attributes to her toolkit apart from her Curse-Warding magic, which is equally terrifying in its own right.
All this, in addition to her tremendous magic power, made her a force to be reckoned with — kind of like a video game boss who has multiple phases, powers, and the notorious ability to regenerate whenever her HP bar hits zero.
Megicula's raw power and tactical capabilities are nothing to scoff at either, with her final battle in the series showcasing just how versatile and calculating she is. After all, it took a coordinated effort from Noelle, Charlotte, Rill, Gadjah, Nozel, and even the dying Lolopechka to finally bring her down for good.
Final Thoughts
To conclude, Megicula is undeniably one of the strongest Devils in Black Clover. Apart from having multiple magic attributes at her disposal, she's intelligent, ruthless, and frighteningly unpredictable in her fights. Despite making a late appearance in the series, she left a lasting impact on both the narrative and the fanbase.