Marids are the most powerful type of demon commonly summoned by magicians. Marids usually take two strong magicians to summon, though they may be manageable for a single especially strong magician. Very powerful entities and magical objects are usually described as having the strength of several marids. The magician Jessica Whitwell is the only magician known to have killed a marid. Jessica used a Void to suck in the marid and kill it.
Throughout the Bartimaeus books, marids are constantly mentioned as incredibly powerful, and in the first book, it is mentioned by Nathaniel that it would generally take two magicians to summons one. They are also mentioned as being too powerful for even multiple djinni to handle, stated by Bartimaeus as able to absorb multiple attacks without trouble, swell to the size of a giant and eat you like carrots without any trouble. It is notable that even Bartimaeus swallows his pride when dealing with these fearsome beings, and when the first marid in the series is shown, Bartimaeus began groveling for his life.