Caelum itself is a weapon
Caelum the "Chisel" is a Silver Keys' Celestial Spirit that can transform between three things for its master. Its key was owned by Angel during the Oración Seis arc and is now currently owned by Kilik Gopher. Apparently, it is only an item and not a sentient being.
Caelum, at its default form, is shaped like a sphere with a halo-like circle at top. It is made of metal and has lines tracing around its shape. The center of Caelum's default body is in a darker shade of gray than the rest of its body and there is a small white circle with a smaller light green circle inside, giving it the appearance of an eye. Caelum can expand its body so that it takes on different forms (sword, flight and cannon are so far the only known forms), but the main circle from its default body remains.
Oración Seis arc
Caelum is summoned to do battle against Lucy Heartfilia. As it is summoned, it fires a blast at Aries and Loke despite Aries being on its side. Gemini then uses Caelum in sword form to attack Lucy but when Gemini refuses to hurt Lucy because Gemini realized Lucy's love for her Celestial Spirits, Angel takes Caelum to attack Lucy herself, however she misses her shot against Lucy.
Magic and Abilities
Cannon Form: In this form it appears as a high tech cannon that can launch powerful blasts of energy. Firing Caelum might require focus or stamina from it's summoner, as Angel was shown to be too injured and beaten up to think straight after getting hit by Uranometria, and the energy beam she fired curved around Lucy and hit the raft Natsu was on instead. Despite of this Angel fell with no more power to fight.Flight Form: In this form it appears as a simple winged sphere which seems to make it the basis for the other forms. This form has yet to be seen using any attacks and it probably has none. Though it's likely this is the only form shown that can move considering it's the only one that looks like it could, but it hasn't been seen doing this yet, so there is no proof of this.
Sword Form: This form has the "chisel" part of the sphere enlarged so much it looks like the Chisel is the blade of a sword with the Sphere part being it's hilt. This form is in fact the first evidence of some stellar spirits having no ability to resist their masters and it seems Caelum even lacks a personality or speech abilities which makes it very different from other Celestial Spirits shown so far.
Immortality: As a Celestial Spirit, Caelum cannot die, unless it is forced to stay in the Human World somehow.