Claiming is the term for a Caster choosing Light or Dark depending on their true nature and personality. Casters are only claimed when the moon is out and if there is no moon out, then they aren't Claimed.
All Casters, except the Duchannes family, have free will in their Claiming whether they choose Light or Dark. Genevieve Duchannes used the Book of Moons to try to resurrect her lover, Ethan Carter Wate, and in return she made herself turn Dark and gave up the free will of her descendants in the Claiming. Lena Duchannes is the only member from the Duchannes family that had free will in her Claiming as she is the second Natural.
After a Light Caster is Claimed, they feel warm and positive like the sun itself was taking over them entirely. They also get fluorescent, glowing green eyes that changes shade depending on their mood. After a Dark Caster is Claimed, they feel cold and powerful. They will become sociopathic; no longer having empathy and remorse, and are incapable of love. They are taken by negative emotions. Ridley says going Dark changes the way you feel about people. When a Dark Caster turns Dark, they sense the feelings they had, the things they loved, but those feelings are distant.
During a Dark Caster's Claiming, they start to shake violently and a devastating cold tears through their veins. They also experience unbearable physical pain, as if their body is tearing itself from the inside, and their vision is then clouded by a golden haze. In the deleted scene of the film adaptation, Ridley described a Dark Caster's Claiming as feeling hot inside that doesn't burn them.
In the film adaptation, only female Casters go through the Claiming. A female Caster's true nature chooses Light or Dark for her depending on her true nature. The Casters that were Claimed glowed.
Lena claimed herself both Light and Dark causing the Order to break. Lena's claiming had consequences: If she claimed herself Light, all of the Dark Casters in her family will die, and vise versa. Claiming herself to both the Light and the Dark required the sacrifice of one Dark Caster and one Light Caster from her family. Larkin Kent was sacrificed from the Dark and Twyla Valentin from the Light.
Blinding light surged from the moon, like a sonic boom. Moonlight flooded the cave, pure and blinding. Lena's eyes glowed yellow and green. She flung her head back to face the moon. Her body twisted, her feet hovering above the stone. Above her, the brightness of the moon began to vibrate, the light pulling, until the whole cave was one ball of light. The moon continued to swell. Like a moment from a dream, the moon split into two halves, dividing in the sky directly over where Lena stood. The moonlight behind her seemed to form a giant, luminous butterfly, with two brilliant, glowing wings. One green, one gold.
Passages in the Book of Moons
"...Until the darkening bringes the tyme of clayming, at the sixteenth moone, when the person of powere has the freedom of wille & agencie to caste the eternal choice, in the end of daye, or the last moment of the last oure, under the clayming moone..."
"...The clayming, once bound, cannot be unbound, the choice, once cast, cannot be recast. A person of powere falles into the great darkening or the great light, for all tyme. If tyme passes & the last oure of the sixteenth moone flees unbound, the order of things is undone. This must not be. The Booke will binde that whiche is unbond, for all tyme."
"Amongst personnes of powere, there being twinne forces from whyche spring all magick, the darknesse & the light."
"At the tyme of the clayming, the truthe will be made manifeste. What appears darkness maye the greatest light, what appears light maye be the greatest darknesse."
"The nyght of the clayming being the nyght of greateste weaknesse, whenne the darkenesse withinne enjoins the darkenesse withoute & the personne of powere openness to the greate darkenesse, so stripped of protections, bindings & casts of shielde and immunitie, death, at the houre of clayming, is most finale & eternalle..."