Crossroads Demons are entities that make formal agreements or bargains with humans, granting any wish in exchange for claiming their life and soul at a fixed point in the future. Deals made with humans are sealed with a kiss, and contracts are written invisibly on the skin of the person. They tend to have red eyes. When the person dies either before his or her time, or is killed by a hellhound at the end of the appointed period, his or her soul is sent to hell. Terms of the demonic contracts vary, but the person selling his or her soul is usually given ten years to live after the deal is made. A Crossroads Demon is typically summoned by burying a container of ritual items, including a photograph of the summoner, graveyard dirt, a black cat bone, and yarrow at the center of a crossroads. The contract is not usually held by the demon who negotiates the deal. There is a list of strengths and weaknesses a mile long that apply to these creatures as well as other demons. For a full list click here.
Curses are basically what they sound like, a curse put on someone or something that can cause anything from simple bad luck to death. There is no known way of breaking a curse except for getting the person who enabled the curse to cancel it, or possibly killing the cursemaker and hoping the curse dies along with it.
Curse boxes are locked, wooden containers with sigils on the exterior. They are designed to contain the magic or cursed objects to prevent them from causing harm. Cursed objects are created by magic, and can kill their owners. The idea for the curse box may have come from the legend of Pandora's Box, a box that holds all the evil and all the diseases in the world, until - once opened - it releases all this evil upon the world, leaving only Hope behind.
Eve is older than Castiel and was last on Earth 10,000 years ago. She is also known as the Mother of All, which refers to her creation of all supernatural beings. She was in Purgatory, where the souls of the supernatural go after death, until dragons raise her. Eve can, because she is older than angels, inhibit them from using their angelic powers and abilities. She can morph her appearance to match virtually anyone she wants. Eve can easily overpower and kill humans, and she is more powerful than her monster children. She can move extremely fast. She can create new monsters with a touch or a whisper. She has to go through a few iterations - what she calls beta tests - before she perfects her last monster, the Jefferson Starships. Eve can speak to monsters through their minds and control them and can see through the eyes of any monster in the world and hear what they hear. She also has the power of levitation and possession. Her only weakness is Phoenix ash which can kill her.
One of the four archangels, Gabriel lives in Heaven until Michael and Lucifer's constant fighting over the creation of humanity causes him to seek refuge on Earth. He takes an unnamed vessel and hides as a trickster, embracing the lifestyle of a demi-god and meting out "just desserts" to those he felt deserved punishment. During his time on Earth he becomes a friend to some the real pagan gods he encounters; they know him as Loki. He can teleport himself and others to any location, and can also banish or imprison lesser angels, see glimpses of the future, create illusions, including copies of himself, transform objects into something else, and transform the appearance of his vessel and alter reality to create time loops/pocket universes; he also can choose who is aware of what is happening. He can revive the dead and harm other archangels. His weaknesses include Holy fire can bind Gabriel to a fixed location and can kill him if he crosses it. An archangel sword can kill Gabriel, other archangels can harm Gabriel, and is possible to bind Gabriel with magical spells.
The Khan Worm is a monster newly created by Eve after she is released from Purgatory. Although it looks like a cross between a simple slug and a centipede, it is a self-aware, intelligent parasite that can enter a person's ear and take control over their actions as well as access their memories. After it leaves its host's body, a black, gooey substance is deposited in the person's ear. Other names: Herpe and Sluggo. It can control their victims' actions and access their memories once it has entered their body through the ear. It increases the strength of the host bodies they possess and can reanimate a host bodies corpse if it has been killed. A current of electricity can stun or kill a Khan Worm depending on its intensity and duration.
(1) Supernatural Wiki