To Reign in Heaven. The Carolingian, #4

When you're Paladins, there's never a day off. On the eve of his ward's graduation from M. I. T., instead of celebrating, Paladin and warrior Richard Oort finds himself rescuing a young scientist from attack by horrific monsters from an alternate dimension. His ward, Mosi, Paladin-in-training, helps to save the physicist, Fox Erskine. They soon discover why Richard's ancient enemies, the Old Ones, targeted Fox: the Brit is on the verge of developing faster-than-light-speed travel.
Because nothing goes smoothly for Richard, his husband Damon, and Mosi, more mysterious enemies attack the graduation ceremony leaving dozens dead. The coincidence is too great, and it seems the twin attacks may be intended to stop humans from escaping the confines of Earth. When a rocket carrying a critical component is destroyed on the launch pad, and there are attempts to kidnap Richard and Mosi, it seems certain.
While Richard, Mosi and Damon are dealing with multiple crises, Fox discovers that his efforts to develop a workable model for faster than light travel has birthed fully sentient A. I.. With the A. I.'s assistance, what was merely theory becomes reality. But Fox's breakthrough represents a profound danger to the other multiverses, and an Old One arrives, not to kill Richard, but to beg for his help.
Promised the stars, humans will only trigger devastating genocide. Faced with a terrible choice that could cost humanity their future among the stars, Richard considers a desperate step which may well forever trap humanity on Earth -and that will require that Richard make the ultimate sacrifice. But Damon and Mosi may not be willing to let him pay that price.