The Reluctant Queen (The Queens of Renthia #2) by Sarah Beth Durst

After Queen Daleina discovers that she needs an heir more quickly than she thought, she sends her champions out to find powerful young women who could take on that role. Naelin is not young, nor does she think she is powerful, but after Champion Ven sees the depth of her strength, he believes she is the heir Renthia needs, even if she wants no part of her powers.