A Celestial Spirit owned by Angel. Its more of an item than a being that can send strong attacks and can transform into three different things.
Caelum's Gate Key.
Caelum, known as "The Chisel", is a Celestial Spirit that can take on three forms. It's previous owner was Angel, but after her defeat, its contract with her became void. Apparently it is an item, not a sentient being but the anime contradicts this when it shows that Caelum misses Lucy on purpose contrary to Angel's orders. Although Aries, Scorpio and Gemini joined Lucy following Angel's arrest, Caelum did not. No information is given as to what had happened to it after the Oracion Seis Arc.
Created by Hiro Mashima, this spirit takes the form of a chisel because "the chisel" is the modern translation of the constellation Caelum. But its been argued that this lacks accuracy due to the ambiguity of the word Caelum. Although in Latin, Caelum means "the heavens" and "burin" which is an instrument used to engrave on metals, hence the reason why the constellation was named "Caelum", the correct name that Hiro Mashima could have used is "scalprum" which means "chisel" in Latin.
Powers & Abilities
Caelum's Cannon Form.
Cannon Form: Caelum can transform into a high tech cannon that can launch a powerful blast of energy. It seems that in order for it fire, it requires stamina from the summoner as displayed by Angel who was too injured to think straight when Caelum's shot missed Lucy (although the anime hints that this was because Caelum missed on purpose).
Caelum's Flight Form.
Flight Form: In this form, Caelum appears as a winged sphere though it has not been shown to have any offensive abilities, which may mean that it has none. Hence, the purpose of this form is unknown but its theorized that it helps Caelum move more freely although this has not proven.
Caelum's Sword Form.
Sword Form: Caelum's "chisel" part of the sphere is enlarged to the point that it looks like the blade of a sword. In this form, the existence of spirits who lack the abilities to resist their masters is proven as Caelum is used as a weapon without control.