Calapians were a six-headed species that were native to the planet Calapia.
The Thirteenth Doctor described the Calapians as "a wonderful bunch, throw a party at the drop of a hat, six heads, lots of hats". She’d also said that they didn’t like to talk about the ruins, and had never been able to figure out why.
The Death Moon forced the Calapians into shelter when it passed over Calapia every sixty-four years.
When the Thirteenth Doctor visited the planet in the company of Yaz, Ryan, and Graham during a passing of the Death Moon, they too were forced to shelter in a bunker with some Calapians on the planet for three weeks. (PROSE: The Shadow Passes)