The Unicorns of Lazaronia — Laraine Anne Barker

Within moments Esmé felt the full force of a refusal that could break even the strongest mind -- a refusal that turned her power against her with a towering, almost unbelievable rage. The twisting rainbows from her fingertips bent like hairpins, aiming their might at her head. For long moments Esmé had to put all her concentration into forcing them back. But it was no good: she had to drop her hands and command the light to go out.

Mark and Esmé are about to face their final battle against the forces of evil swamping Lazaronia, and Esmé needs to consult The Voice of Judgement. To do this she must command the power of the Tower of Kaleidoscopic Light. But the tower tries to turn her power against her. And the Voice of Judgement rightly refuses to admit her, because for someone still living to enter the presence of the Voice of Judgement is strictly forbidden.