Shadow Kin were capable of producing black scimitars from their bodies. These sabres were also capable of opening space-time rifts, (TV: Co-Owner of a Lonely Heart) which the Shadow Kin used to invade and massacre countless planets, including Rhodia. (TV: For Tonight We Might Die)
King Corakinus plunged his through Rachel Chapman, killing her mercilessly in front of her boyfriend Ram Singh. He later used his scimitar to sever Ram's leg when he confronted him. (TV: For Tonight We Might Die)
After Corakinus and April MacLean began sharing April's heart, (TV: For Tonight We Might Die) she gained the ability to produce them as well, much to her own surprise. She threatened and later attacked her father Huw MacLean with them.
While Huw referred to them as swords, April said they were more like scimitars. April used hers to create a rift in her backyard, and entered it into the Underneath, followed by Ram. (TV: Co-Owner of a Lonely Heart) She used it to kill several Shadow Kin, and to defeat Corakinus in battle and become the new king of the Shadow Kin. She then opened a rift back to Earth. April's connection to Corakinus was subsequently weakened, and her powers went away. (TV: Brave-ish Heart)